SECTION 5

BRAKES

CONTENTS OF THIS SECTION

The brakes used on both front and rear of all models are the Duo-Servo single anchor type which utilize the momentum of the vehicle to assist in the brake application. This self-energizing of self-actuating force is applied to both brake shoes at each wheel in both forward or reverse motion.

Each brake (fig. 1) has one wheel cylinder located near the top of the brake flange plate just below the anchor pin. Each wheel has two shoes -with a pull back spring installed between each shoe and the anchor pin to hold the upper ends of the shoes against the anchor pin when the brakes are released. The lower ends of the shoes are connected by a link and a helical spring. The link is made up of an adjusting screw, riding in a socket at one end, and threaded into a pivot nut at the other. The outer ends of the socket and pivot nut are notched to fit the webs of the brake shoes, providing freedom of motion between the link and the shoes. The spring is stretched from one shoe web to the other, crossing over the notched head of the adjusting screw. It bears against one of the notches in the head, and thus acts as a lock for the adjusting screw. Bonded brake linings are used and brake drums are 11" in diameter. The front brakes are 2" wide while the rear are 1 3/4".

In each brake assembly, the linings for the front and rear shoes differ in length with the secondary facing being 2 1/2" longer than the primary, because in operation, a greater force is applied to the secondary facing than to the primary.

The brake flange plate has six bearing surfaces, three for each shoe, against which the inner surfaces of the shoes bear to maintain alignment. Slightly below the center of each shoe web is a hole through which a hold down pin is inserted. A spring and cap, fitted over the outer end of the pin, holds the shoe against the bearing surfaces. At the top of the brake where the shoes butt against the anchor pin, a guide plate separates the pull back springs from the shoe webs, and assists in keeping the shoes properly aligned. The brake mechanism is effectively sealed against the entrance of dirt or mud by the joint between the brake flange plate and the drum. The outside edge of the flange plate fits over the edge of the drum which has an annular groove located between two flanges.

The outer flange is of a larger diameter than the inner one, so that dirt and moisture which correct in the groove are thrown off the larger flange by the centrifugal force of the rotating drum, thus keeping foreign matter away from the drum-to-flange plate joint.

OPERATION

When the brakes are applied, the pistons in the wheel cylinder, acting on the brake shoes through the connecting links, force the shoes against the drum. Since the shoes float free in the brake, the force of friction between the shoes and the rotating drum turns the entire assembly in the direction of the wheel rotation. The front or primary shoe moves downward, and the back or secondary shoe is carried upward until its upper end butts against the anchor pin. The friction between the moving drum and the stationary shoes now tends to roll both shoes toward the drum with increased pressure. The secondary shoe pivots on the anchor pin at the top, and the primary shoe tends to turn about the adjusting link at the bottom which is held stationary by the secondary shoe. This self energizing effect, greatly increases the pressure of the shoes against the drum and reduces the physical force required on the brake pedal.

Inasmuch as the brake shoes are freely connected at the bottom by the adjusting link, the self-energizing or friction force which is applied to the primary shoe by the brake drum is transmitted to the secondary shoe through the link. The effectiveness of the secondary shoe is nearly doubled, because the total force applying this shoe becomes the sum of the force which is received from the primary shoe and the self energizing effect that is derived from the rotating drum.

When backing the car, the brake action is reversed. The rear shoe becomes the primary shoe and the front shoe becomes the secondary, butting up against the anchor pin during braking and being forced against the drum with great pressure.

Wheel cylinders (fig. 2) are the double piston type permitting even distribution of pressure to each brake shoe. To keep out dust and moisture and to prevent gumming of the brake fluid, both ends of each wheel cylinder are sealed with a rubber boot. The wheel cylinders have no external adjustments.

Operation of the hydraulic system is dependent upon the proper functioning of main and wheel cylinders. The main cylinder (fig. 3) consists of a piston which receives mechanical pressure from the push rod and exerts pressure on the fluid in the lines, building up the hydraulic pressure, which moves the wheel cylinder pistons. The primary cup is held against the piston by the piston return spring which also retains the return valve against its seat. The spring maintains a slight pressure in the lines and in the wheel cylinders to prevent the possible entrance of air into the system. The secondary cup, which is secured to the opposite end of the piston, prevents the leakage of fluid into the rubber boot. The holes in the piston head are for the purpose of allowing the fluid to flow from the annular space around the piston into the space between the primary cup and the check valve, keeping sufficient fluid in the lines at all times. The holes in the valve cage allow the fluid to flow through the cage and around the hp of the rubber valve cup and out into the lines during the brake application. When the brake is released, the lip of the rubber valve cup seals the holes in the valve cage and the valve is forced off its seat, permitting the fluid to return to the main cylinder. A rubber boot that fits around the push rod and over the end of the housing prevents dirt or any other foreign matter from entering the main cylinder.

As the pedal is released, the hydraulic pressure is relieved and the brake shoe retracting springs draw the shoes together, pressing the wheel cylinder pistons inward and forcing the fluid out of the wheel cylinders back into the fines toward the main cylinder. The piston return spring in the main cylinder returns the piston to the pedal stop faster than the brake fluid is forced back into the lines, creating a partial vacuum in that part of the cylinder ahead of the piston. This vacuum causes a small amount of fluid to flow through the holes in the piston head, past the lip of the primary cup and into the forward part of the cylinder. This action keeps the cylinder full of fluid at all times, ready for the next brake application. As fluid is drawn from the space behind the piston head, it is replenished from the reservoir through the inlet or breather port. When the piston is in a fully released position, the primary cup clears the compensator port, allowing excess fluid to flow from the cylinder into the reservoir as the brake shoe retracting springs force the fluid out of the wheel cylinders.

The passenger car parking brake lever is located to the left of the steering column. ( fig. 5 ) A cable type linkage, directed over two pulleys, onnects this lever to an idler lever, mounted in a bracket bolted to the under body approximately in the center of the vehicle. The idler lever serves to increase the leverage of the system and to connect the forward pull cable to the hand brake equalizer. The equalizer, attached to the idler lever, holds a one piece cable that is attached to each rear wheel brake. The equalizer permits an even pull on each brake.

Care, Maintenance and Adjustments

Care, Maintenance and Adjustments General notes
Hydraulic Brake Fluid
Bleeding Hydraulic System
Toe Push Rod to Main Cylinder Clearance
Hydraulic Brake Tubing
Hydraulic Brake Adjustment
Parking Brake Adjustment
Foot Pedals Clutch and Brake

Care, Maintenance and Adjustments General notes

In any service operation it is extremely important that absolute cleanliness be observed. Any foreign matter in the system will tend to clog the lines, ruin the rubber cups of the main and wheel cylinders and cause inefficient operation or even failure of the braking system. Dirt or grease on a brake lining will cause that brake to grab first on brake application and fade out on heavy brake application.

Hydraulic Brake Fluid

Only G. M. Hydraulic Brake Fluid Super No. 11 should be used when bleeding brakes. This brake fluid is satisfactory for any atmospheric temperature hot or cold and has all the qualities necessary for satisfactory operation, such as a high boiling point to prevent evaporation and tendency to vapor lock and remains fluid at low temperatures.

In the event that improper fluid has entered the system, it will be necessary to-

1. Drain the entire stem.
2. Thoroughly and vigorously flush the system with clean alcohol, 188 proof, or a hydraulic brake system cleaning fluid such as "Declene."
3. Replace all rubber parts of the system, including brake hoses.
4. Refill the system with G.M. Hydraulic Brake Fluid Super No. 11.

General Notes
Pressure Bleeding
Manual Bleeding

Bleeding Hydraulic System General Notes

The hydraulic brake system must be bled whenever a pipe line has been disconnected, when a leak has allowed air to enter the system or at any time the system has been opened. The system must be absolutely free from air at all times.

Bleeding should be done on the longest line first and the proper sequence to follow is left rear, right rear, right front and left front.

NOTE: The proper bleeding sequence for all units equipped with Hydrovac is Hydrovac slave cylinder, Hydrovac valve bleeder valve, left rear, right rear, right front and left front.

Bleeding of brake lines may be accomplished by one of two methods: Pressure or Manual.

Pressure Bleeding

1. Clean all dirt from top of main cylinder and remove filler plug.
2. Connect hose from bleeder tank to main cylinder filler plug opening and open valves at both ends of hose. NOTE: Make sure fluid in tank is up to petcock above outlet and that tank is charged with 1 0 to 20 pounds air pressure.
3. Remove bleeder valve screw and screw bleeder hose into bleeder valve, placing other end of hose in a container having sufficient fluid to cover end of hose (fig. 5).
4. Open bleeder valve by turning 3/4 turn in a counterclockwise direction and watch flow of fluid at end of bleeder hose.
5. Close bleeder valve tightly as soon as bubbles stop and fluid flows in a solid stream.
6. Remove bleeder hose and install bleeder valve screw in bleeder valve.
7. Repeat above operations at each wheel.

1. Clean all dirt from top of main cylinder and remove filler plug.
2. Install Tool 0713 (fig. 6).
3. Remove bleeder valve screw and screw Tool 0628 into bleeder valve, placing other end of hose in a container having sufficient fluid to cover end of hose (fig. 5).
4. Open bleeder valve by turning 3/4 turn in a counterclockwise direction.
5. Depress foot pedal by hand, allowing it to return slowly. Continue this pumping action to force fluid through the lines and out at the bleeder drain, carrying with it any air in the system.
6. Close bleeder valve tightly as soon as bubbles stop and fluid flows in a solid stream.
7. Remove bleeder hose and install bleeder valve screw in bleeder valve.
8. Repeat above operations at each wheel.

This clearance very seldom needs adjustment. The brake pedal has a definite stop which is permanent and not adjustable. This stop consists of a rubber bumper at the release end of pedal travel. Before adjusting push rod to main cylinder clearance, make sure brake pedal returns to the fully released position freely, with no binding, and that the pedal retracting spring has not lost its tension.

1. Brake pedal clevis should be adjusted to give barely perceptible end play between main cylinder piston and the brake push rod.
2. Loosen check nut on the clevis.
3. Turn push rod in proper direction to secure the proper adjustment, barely perceptible movement of pedal before contact of push rod and main cylinder piston (1/8" to 3/8" pedal movement should be felt).
4. Tighten check nut on clevis.

Hydraulic Brake Tubing
Double Lap Flaring

Hydraulic Brake Tubing

Hydraulic brake tubing used on all models is a double layer annealed steel, copper coated and tin plated tubing which resists corrosion and also stands up under the high pressures which are developed when applying the brakes. All models use 3/16" tubing, except on the brake main cylinder pipe and front crossover pipe which are 1/4". In making up hydraulic brake pipes, it is important that the proper size flaring tool be used to flare the ends of the tubing for the compression couplings. Unless the tubing is properly flared, the couplings will leak and the brakes will become ineffective.

CAUTION: When necessary to replace broke tubing, always use special metal tubing which is especially designed to withstand high pressure and resist corrosion. For this reason, ordinary copper tubing is not satisfactory and should not be used.

This safety steel tubing must be double-lap flared at the ends in order to produce a strong leak-proof joint.

The brake tube flaring Tool 8051 ( fig. 7) is used to form the double-lap flare.

Figure 8 shows two pieces of tubing-line with a single-lap flare "A" and the other with a double-lap flare "B". It will be noted that the single-lap flare split the tubing while the one shown in "B" has a heavy, well-formed joint.

The following procedure should be followed in making up hydraulic brake pipes:

Double Lap Flaring

1. Clamp the tubing in the proper size die blocks with the flat ends of the blocks toward the end of the tubing to be cut off. Cut the end of the tubing flush and square. Using a mill file, dress tubing and square ends.
2. Remove the tubing from the die block, and deburr the inside and outside edges.
3. Install compression couplings on tubing and dip end of tubing to be flared in hydraulic fluid. This lubrication results in better formation of the flare.
4. Place one-half of the die blocks in the tool body with the counterbored ends toward the ram guide. Now lay the tubing in the block with approximately 1/2" protruding beyond the end. Fit the other half of the block into the tool body, close the latch plate and tighten the nuts "finger-tight."
5. Select the correct size upset flare punch. One end of this punch is counterbored or hollowed out to gauge the amount of tubing necessary to form a double lap flare. Slip the punch into the tool body with the gauge end toward the die blocks. Install the ram; then tap lightly until the punch meets the die blocks and they are forced securely against the stop plate (fig. 9).
6. Using the supplied wrench, draw the latch plate nuts down tight to prevent the tube from slipping. Tightening the nuts alternately (beginning with the nut on the closed side) will prevent distortion of the plate. Remove the punch and the ram. Now reverse the punch and put it back into the tool body. Install the ram and tap it lightly until the face of the upset flare punch contacts the face of the die blocks (fig. 10). This completes the first operation. Remove the ram and the punch.
7. To complete the flare, insert the finish flare punch and the ram into the tool body. Tap the ram until a good seat is formed (fig. 11).

NOTE: The seat should be inspected at intervals during the finishing operation to avoid over-seating.

Hydraulic Brake Adjustment

Hydraulic brake adjustment is confined to a single operation on each brake assembly. A spring snap cover plate is pried from the back of the flange plate, exposing a hole through which Tool 4707 is inserted to turn the single adjusting screw.

Adjustment Front or Rear

1. Jack all wheels clear of floor.
2. Loosen the check nut at the brake cable equalizer to remove tension from brake cable. NOTE: If cable has been adjusted too short, the rear brake shoes will be forced away from the anchor pins in brake release position, making correct shoe adjustment impossible.
3. Remove adjusting hole cover from brake flange plate. Expand brake shoes by turning adjusting screw with Tool 4707 until a light uniform drag is felt on the brake drum (fig. 12). NOTE: Moving the outer end of tool toward center of wheel, expands the shoes.
4. Turn adjusting screw back, to contact brake shoes, 7 notches to insure running clearance, after light uniform drag is felt.
5. Repeat the above operations 3 and 4 at each wheel and replace hole covers.
6. After the hydraulic brakes are adjusted, adjust the parking brakes as outlined under, "Parking Brake Adjustment."
7. Lower car to floor and test brakes.

Parking Brake Adjustment
Hand Brake Service

Parking Brake Adjustment

The parking brake must be adjusted each time the hydraulic service brakes are adjusted. When making a parking brake adjustment, the service brake must be properly adjusted first as a base for the parking brake adjustment.

1. Jack up both rear wheels.
2. Check the clearance between the idler lever and mounting bracket at the equalizer to be sure the front cable is correctly adjusted. (This clearance from the rear of the idler lever to mounting bracket is approx. 1/4 inches). To correct front cable adjustment, the equalizer must be disconnected, the check nuts on the forward cable tightened so the correct clearance may be established (ordinarily this clearance will be correct even after long periods of service). fig. 13.
3. Pull the hand brake out 7 "clicks" not notches. (They are not the same.)
4. Loosen the forward check nut on the equalizer and tighten the rear one until there is a slight drag on both rear wheels when rotating the drums.
5. Tighten the check nuts securely.
6. Set the parking brake lever back to 2 clicks from full release position, at which point no brake shoe drag should be felt.

If complete release of the hand brake lever is not obtained, unless the lever is forcibly returned to its released position, it indicates a need for forward cable adjustment or that there is a bind in the lever or cable, or a weak, broken, or missing equalizer return spring. Check hand brake lever assembly for free operation. If operation is sticky or a bind is experienced, correct as follows:

1. Check clearance from idling lever to mounting bracket and correct if necessary according to Step No. 2, "Parking Brake Adjustment."
2. Clean and lubricate forward brake cable pulleys.
3. Inspect brake handle and shaft for straightness (replace if necessary).
4. Clean and lubricate parking brake shaft.

Hand Brake Cable Removal

1. Remove clevis pin at end of forward brake cable.
2. Remove clinch nuts and clevis.
3. Remove the bolts from each of the cable pulleys.
4. Pull out the parking brake lever to the end of its travel.
5. Remove the cable from the parking brake lever by pulling the cable ball out of the recess in the brake lever.

Hand Brake Cable Installation

1. Insert the knob end of the hand brake cable through the two cable pulley housings and floor board from underneath the vehicle. NOTE: The cable pulleys must be removed to provide clearance for the cable ball to pass through the pulley housings.
2. Lubricate and install the 2 cable pulleys.
3. Insert the cable knob into the recess on the hand brake lever with the hand brake lever pulled out to the end of its travel.
4. Push the parking brake lever in to the full released position.
5. Install the clevis to the cable.
6. Install the clevis to the parking brake equalizer arm with the clevis pin.
7. Install a new cotter pin in the clevis pin.
8. Adjust the parking brake system as outlined under "Parking Brake Adjustment."

Front Brake Cable Adjustment

1. Loosen the check nut on the clevis at the idler arm.
2. Turn adjusting nut to just remove slack with lever in released position.
3. Tighten check nut securely.

If the cable is bound up or sticky, free up the two pulleys by lubricating them with light oil or replace the pulleys.

Clutch and Brake Pedals

Fig. 14

Integral clutch and brake pedal assemblies are provided on passenger models equipped with syncro-mesh transmissions. The same brake pedal assembly is used on models equipped with Powerglide or Turboglide. In either case the pedal assembly is suspended from a brace fastened from the instrument panel to dash panel. Both pedal assemblies are removable for service. The brake pedal assembly is mounted to the panel brace utilizing nylon bushings on the brake shaft which is installed through the opening provided on the panel brace. Since the brake pedal shaft is hollow, the clutch pedal shaft is conveniently installed through the brake pedal shaft, again using nylon bushings and adjustable linkage. A brake pedal coil spring used in the assembly returns the pedal to its original position after brake application. A clutch pull back spring permits smooth light operation to engage or disengage the clutch. When the pedals are not in use, spring tension from the springs holds the pedal arms back against rubber stops. The rubber stops are removable for replacement.

Clutch Pedal Assembly

Removal

1. Disconnect the clutch push rod from the push rod lever.
2. Release the pull back spring tension by pushing the clutch pedal down to the end of its forward travel against the floor board.
3. Remove the lower bolt after securely tightening the upper bolt.
4. Loosen the upper bolt while slowly relieving pull back spring tension.
5. Pull pedal back against the stop bracket.
6. Remove the pull back spring.
7. Remove the clutch push rod lever retaining bolt.
8. Remove the clutch push rod lever and spacer.
9. Remove the clutch pedal shaft assembly and bushing.

Inspection

1. Clean all metal parts with a good cleaning solvent.
2. Wipe the nylon bushings clean with a clean cloth.
3. Inspect the clutch pedal shaft for damage to the serration on the end of the shaft.
4. Inspect the clutch pedal lever for wear, bend, or damaged teeth.
5. Inspect the nylon bushings for wear.
6. Replace all worn or damaged parts.

Installation

1. Install the clutch pedal pull back spring lever to the clutch pedal with the upper retaining bolt only.
2. Tighten the retaining bolt until the lock washer is partly compressed and there is a heavy drag between the bracket and pedal arm with hand pressure.
3. Lubricate the nylon bushings with lubriplate or a similar lubricant and install one on the clutch pedal shaft with the shoulder of the bushing flush against the pedal arm.
4. Lubricate the clutch pedal shaft with lubriplate or a similar lubricant and install the assembly through the brake pedal sleeve.
5. Install the second nylon bushing on the clutch pedal shaft and push it into the brake pedal sleeve, lubriplate pull back spring ends.
6. Connect the short hook end of the clutch pull back spring to the hook on the instrument panel brace.
7. Connect the other end of the pull back spring to the return spring link.
8. Allow the clutch pedal to return back to the end of its return travel.
9. Insert a large screwdriver or other suitable tool under the panel brace and over the clutch pull back spring lever.
10. Pull down on the spring lever with the tool until the hole in the clutch pedal pull back spring lever is aligned with the tapped hole in the clutch pedal arm.
11. Install the lower bolt, flat washer and lock washer.
12. Install the steel spacer over the serrated end of the clutch shaft next to the nylon bushings. CAUTION: Be sure the nylon bushing is correctly seated.
13. Install the push rod lever on the clutch shaft with the arm next to the spacer.
14. Install the push rod lever retaining bolt, lock washer and nut.
15. Connect the clutch pedal push rod lever and push rod with the spring, flat washer and cotter pin.

Brake Pedal Removal

1. Remove clutch pedal assembly as outlined under "Clutch Pedal Assembly."
2. Remove the brake pedal rubber bumper stop.
3. Remove the clevis pin from the main cylinder push rod clevis.
4. Remove the bolt retaining the brake pedal sleeve to the instrument panel brace assembly.
5. Remove the brake pedal sleeve to the pedal and instrument panel brace.
6. Remove the pedal assembly from the vehicle. CAUTION: Exercise care in removing the pedal assembly in order to avoid damaging the ends of the nylon bushings.
7. Remove the nylon bushings from each end of the brake pedal hub.
8. Remove the pedal return spring.

Inspection

1. Clean all metal parts with a good cleaning solvent.
2. Wipe the nylon bushings clean with a dry clean cloth. CAUTION: Nylon bushings should not be treated with cleansing agents of any nature.
3. Inspect the bushings for their respective fit on the brake pedal sleeve and in the bore of the brake pedal hub.
4. Inspect the sleeve and hub for wear.
5. Replace the bushings, the sleeve, or both as the case may require.

Assembly

1. Lubricate the nylon bushings, the sleeve, and the pedal hub with lubriplate or a similar lubricant.
2. Install the nylon bushings in the brake pedal hub.
3. Install the pedal return spring on the pedal hub.
4. Position the pedal assembly in the correct location in the instrument panel brace and engage the return spring on the panel brace with the main cylinder clevis aligned with the pedal assembly.
5. Install the pedal sleeve thru the instrument panel brace and pedal bushings.
6. Install the pedal sleeve retaining bolt, nut, and lock washers.
7. Install the rubber pedal bumper stop.
8. Install the main cylinder push rod clevis pin and cotter pin.
9. Check the main cylinder to push rod clearance and adjust if necessary as outlined in push rod to main cylinder clearance.

NOTE: Check stop light switch position and adjust if necessary so that electrical contact is made when the pedal is depressed 5/8 inch.

Major Service Operations

Major Service Operations General Notes
Brake Shoe Replacement
Main Cylinder
Wheel Cylinder
Brake Drums, Removal, Inspecting and Reconditioning
Cleaning

Major Service Operations General Notes

In all cases of brake complaints denoting actual brake lining or shoe failure, the brake drums should be removed and before disassembly of the shoes from the flange plate, all linings should be inspected for wear, improper alignment causing uneven wear and oil and grease on the linings. If any of these conditions exist, it will be necessary to replace the shoes. If, in checking the linings, it is noticed that they have the appearance of being glazed, this is a normal condition with the hard type lining used. Do not use a wire brush or an abrasive on the lining to destroy this glazed surface as it is essential for proper operation. When brake lining replacement is necessary, all shoes and linings should be replaced. In no case should a single lining and shoe be replaced; however, in exceptional cases, it may be satisfactory to replace the shoes and linings on both front or both rear wheels.

Brake Shoe Replacement

Removal
Installation

Removal

1. Raise vehicle and place on stand jacks.
2. Loosen check nuts at forward end of parking brake cable sufficiently to remove all tension from brake cable.
3. Remove rear brake drums and front hub and drum assemblies. NOTE: Since boots are recessed in grooves on wheel cylinders to prevent pistons from leaving cylinders, it is not necessary to install wheel cylinder clamps when brake shoes are removed; however, brake pedal must not be depressed while drums are removed.
4. Unhook brake shoe pull back springs from anchor pin using Tool KMO 526-A (fig. 15). NOTE: Be certain that the primary and secondary springs are kept separate as they have unequal tension. The spring with the most tension is always the secondary.
5. Remove brake shoe hold down pins and springs using a pair of needle nosed pliers (fig. 16).
6. Spread shoes to clear wheel cylinder connecting links and remove shoes from backing plate (fig. 17).
7. Separate the brake shoes by removing adjusting screw and spring.
8. Remove parking brake lever from secondary brake shoe (rear only).
9. Clean all dirt out of brake drum using care to avoid getting dirt into front wheel bearings. Inspect drums for roughness, scoring or out-of-round. Replace or recondition drums as necessary.
10. Inspect wheel bearings and oil seal and replace any necessary parts.
11. Carefully pull lower edges of wheel cylinder boots away from cylinders and note whether interior is wet with brake fluid. Excessive fluid at this point indicates leakage past piston cups requiring overhaul of wheel cylinder. NOTE: A slight amount of fluid is nearly always present and acts as lubricant for the piston.
12. If working at rear wheels, inspect backing plate for oil leakage past axle shaft oil seals. Install new seals if necessary.
13. Check all brake flange plate attaching bolts to make sure they are tight. Clean all rust and dirt from shoe contact faces on flange plate, using fine emery cloth (fig. 18).

CAUTION: Make certain that when replacing with new shoe and lining assemblies, that the latest type 1957 linings are used. Otherwise, serious fade or premature failure may occur.

1. Inspect new linings and make sure there are no nicks or burrs or bonding material on shoe edge where contact is made with brake flange plate or on any of the contact surfaces. NOTE: Keep hands clean while handling brake shoes. Do not permit oil or grease to come in contact with linings.
2. If working on rear brakes, lubricate parking brake cable.
3. On rear brakes only, lubricate fulcrum end of parking brake lever and the bolt with Bendix or Delco brake lube or Lubriplate, then attach lever to secondary shoe with bolt, spring washer, lockwasher, and nut. Make sure that lever moves freely.
4. Lubricate threads and socket end of adjusting screw with Bendix or Delco brake lube or Lubriplate.
5. Connect brake shoes together with adjusting screw spring then place adjusting screw, socket and nut in position. CAUTION: The socket and adjusting screw must be adjacent to the primary shoe (front) on the brakes an the left side and adjacent to the secondary shoe (rear) on the brakes on the right side.
6. Attach brake shoes to brake flange plates with the hold down pins and springs using a pair of needle-nosed pliers; at the same time engage shoes with wheel cylinder connecting links. The primary shoe (short lining) goes forward.
7. On rear brakes, connect cable to parking brake lever and install strut between lever and primary shoe as installation is made.
8. If old brake pull back springs are nicked, distorted, or if strength is doubtful, install new springs. Install guide plate over anchor, hook springs in shoes and using Tool KMO526-A, install spring connected to primary shoe over anchor ( fig. 19), and then spring connected to secondary shoe over anchor.
9. Pry shoes away from backing plate and lubricate shoe contact surfaces with a thin coating of Bendix or Delco brake lube or Lubriplate. On rear wheels, sparingly apply this same lubricant where brake cable contacts brake flange plate. CAUTION: Be careful to keep lubricant off facings.
10. Install brake drums. If working on front brakes, lubricate and adjust wheel bearings and install front and rear wheel and tire assemblies. Remove adjusting hole covers from backing plates.
11. Adjust all brakes and brake cables as outlined under Care, Maintenance and Adjustments.

NOTE: The fixed anchor pins have eliminated need for anchor pin adjustment.

Main Cylinder

Removal

Fig. 20

1. Disconnect hydraulic line from end of cylinder.
2. Remove bolted brass fitting.
3. Remove the four retaining nuts and lock washers holding main cylinder to the front of the dash panel and remove the cylinder from the vehicle.

Disassembly-Fig. 21 Main Cylinder Construction Details

1. Remove the end plug and valve seat washer.
2. Remove the valve seat washer from the button on the end plug (fig. 22).
3. Remove the valve assembly and spring.
4. Remove the main cylinder boot.
5. Push out the primary cup, piston, and secondary cup by pushing out to the end of the cylinder with a suitable tool.
6. Remove the snap ring and piston stop from front of cylinder.
7. Remove filler plug.

Inspection

1. Wash all parts in clean alcohol. Make sure that compensating port in main cylinder body and bleeder holes in piston are clean and open. NOTE: Before washing parts, hands must be clean. Do not wash hands In gasoline or oil before cleaning parts. Use soap and water to clean hands.
2. Inspect cylinder bore to make sure it is smooth.
3. Inspect primary and secondary cups, valve and valve seat for damage or swelling, Swelling of rubber parts is due to the use of improper brake fluid or washing parts in gasoline or kerosene. NOTE: The primary cup has a brass support ring vulcanized in its base to prevent It from imbedding in the bleeder holes during braking action.
4. Check piston fit in cylinder bore ( fig. 23). The clearance between piston and wall of the cylinder should be from .001"-.005".
5. Check clearance between the edge of the primary cup and the center of the compensating port, (fig. 24). To check this clearance, proceed as follows:
   • Install piston stop and lock in place with snap ring.
   • Assemble secondary cup on piston and install assembly in body. Place primary cup in the body with the flat side of the cup against the piston.
   • Push the piston cup against the piston stop and check clearance between edge of primary cup and center of the compensating port. This clearance should be a minimum of .035". If clearance is less than .035", the primary cup must be replaced. NOTE: This check is made easiest by using a wire Inserted through the reservoir of the body and extending Into the piston chamber ( fig. 25).
6. After clearance is checked, again completely disassemble main cylinder.

Assembly

Whenever a hydraulic brake main cylinder is overhauled, care must be taken to reassemble the valve and seat correctly. Improper assembly of the check valve seat rubber washer will result in its distortion. When the check valve seat is distorted, there will be no check valve seal and there will be a loss of brake pedal travel, also, the pedal will have to be depressed or pumped one or more times before actual car braking occurs.

1. Install the piston stop and snap ring in the front of the main cylinder.
2. Dip the rubber cups and valve in hydraulic brake fluid.
3. Assemble secondary cup to the piston and install assembly in the body so that the bleeder hole end of the piston will be toward the end plug when the assembly is complete.
4. Place the primary cup in the body with the flat side against the piston.
5. Install the piston spring.
6. Install the valve assembly.
7. Dip a new check valve seat washer in genuine hydraulic brake fluid and assemble over the spring.
8. Assemble a new gasket over the end plug and screw the plug, valve seat washer, and gasket into the main cylinder body and tighten securely.
9. Install the rubber push rod boot, making sure this seal is tight on the body. This seal must be tight to keep water and other foreign matter from entering the main cylinder through the pedal stop.
10. Install the brass block, washers and bolt.

Installation

1. Install the main cylinder in place on the dash panel and tighten the mounting nuts securely. Make certain the push rod clevis is straddling the brake pedal.
2. Install clevis pin through clevis and brake pedal and insert cotter pin.
3. Connect hydraulic brake line to cylinder.
4. Refill main cylinder and bleed all brake lines as outlined in this section.
5. Adjust brake pedal clevis as outlined under Push Rod to Main Cylinder Clearance.

Removal

1. Raise vehicle and place on stand jack.
2. Remove wheels. Back off brake adjustment and remove drums NOTE: Front drums are non-demountable and are removed with front wheel hubs. Rear brake drums may be removed by removing rear wheel retaining nuts.
3. Disconnect brake system wheel cylinder pipe or hose from fitting at flange plate.
4. Disconnect brake shoe retracting spring from brake shoes.
5. Remove two capscrews which hold rear wheel cylinder to brake flange plate and remove rear wheel cylinder.
6. Remove anchor pin which holds front wheel cylinder to flange plate and remove front wheel cylinder.

Disassembly-Figure 26 shows construction details

1. Remove the cylinder boots.
2. Remove the pistons, rubber cups and spring.
3. Wash all parts in clean alcohol

NOTE: Before washing parts, hands must be clean. Do not wash hands in gasoline or oil before cleaning parts. Use soap and water to clean hands.

Inspection

1. Inspect cylinder bore, making sure that it is smooth. A scored or damaged cylinder must be replaced.
2. Check rubber cups for damage or swelling due to improper brake fluid. Replace the cups when necessary. Improper brake fluid will cause the cups to swell as much as 40 per cent.
3. Check fit of the piston in the cylinder bore, using a feeler gauge (fig. 27). This clearance should be from .002" - .004".

Assembly

1. Dip pistons and rubber cups in hydraulic brake fluid.
2. Place the spring in the center of the housing.
3. Install rubber cups at each end of spring with cupped side toward spring.
4. Install pistons with flat side of pistons against flat side of rubber cups.
5. Replace boots.

Installation

1. Mount front wheel cylinders to the brake flange plate by installing the threaded anchor pin (Fig. 28) through the wheel cylinder housing and tighten to 65 foot pounds with a torque wrench (Fig. 29). CAUTION: Peon over the flat washer on the anchor pin to prevent the anchor pin from loosening during operation.
2. Mount the rear wheel cylinder to the brake flange plate, install two cap screws, tighten securely and install the two connecting links.
3. On both front and rear, replace the brake shoe retracting springs.
4. Connect the hose or line to the wheel cylinder.
5. Install the rear brake drums and install the wheels.
6. Install front hub and drum and adjust wheel bearings as outlined in Section 3.
7. Bleed all brake lines.
8. Adjust brakes as outlined in Section 5.

Front brake drums are the non-demountable type, that is, they cannot be removed without removing the hub. Whenever this type drum is removed, wheel bearings must be adjusted as outlined in Section 3. Rear brake drums are demountable, that is, they may be removed without removing the axle shaft.

Removal

1. Jack up front end of vehicle and remove wheel.
2. Remove hub and brake drum assembly.
3. Remove brake drum from hub, as outlined in Section 3.
4. Jack up rear end of vehicle and remove wheel.
5. Remove brake drum from flange of axle shaft.

Inspecting and Reconditioning

Whenever brake drums are removed they should be thoroughly cleaned and inspected for cracks, scores, deep grooves, and out-of-round. Any of these conditions must be corrected since they can impair the efficiency of brake operation and also can cause premature failure of other parts.

Smooth up any slight scores by polishing with fine emery cloth. Heavy or extensive scoring will cause excessive brake lining wear and it will probably be necessary to rebore in order to true up the braking surface.

An out-of-round drum makes accurate brake shoe adjustment impossible and is likely to cause excessive wear of other parts of brake mechanism due to its eccentric action.

A drum that is more than .010" out-of-round on the diameter is unfit for service and should be rebored. Out-of-round, as well as taper and wear can be accurately measured with an inside micrometer fitted with proper extension rods.

If drum is to be rebored for use with standard size brake facings which are worn very little, only enough metal should be removed to obtain a true smooth braking surface.

If drum has to be rebored more than .010" over the standard diameter, it should be rebored to .060" oversize and the brake facing should be replaced with .030" oversize facings.

A brake drum must not be rebored more than .060" over the maximum standard diameter, since removal of more metal will effect dissipation of heat and may cause distortion of drum. Chevrolet brake facing is not furnished larger than .030" oversize and this will not work efficiently in drums bored more than .060" oversize.

Brake drums may be refinished either by turning or grinding. Best brake performance is obtained by turning drums with a very fine feed. To insure maximum lining life, the refinished braking surface must be smooth and free from chatter or tool marks, and run-out must not exceed .005" total indicator reading.

Cleaning

New brake drums in parts stock are given a light coating of rust proofing oil to prevent the formation of rust on the critical braking surfaces during the time that the drums are in storage.

This rust proofing oil must be carefully removed before the drum is placed in service to prevent any of this oil from getting on the brake shoe facings, which might cause an extreme brake grab condition.

It is recommended that naphtha or carbon tetrachloride be used to clean the oil from the braking surface of the new brake drums before they are placed in service to insure the cleanest possible surface.

Gasoline or Kerosene should not be used as there is danger that a portion of the diluted oily substance may be left on the braking surface that may later cause difficulty. Installation

1. Make sure mating surfaces of hub, drum and oil deflector are clean and smooth and assemble front drum as outlined in Section 3.
2. On front, install drum and hub assembly to wheel spindle and adjust bearings as outlined in Section 3.
3. On rear, assemble drum over axle shaft studs.
4. Replace wheel assembly, adjust brakes and lower vehicle to floor.

Model
    All Models
        Front
        Rear


Brake Size
    Front----- 11
    Rear------ 11


Lining
    Thickness    
          Front----- .187-.194
          Rear------ .187-.194
    Width
          Front----- 2"
          Rear------ 1 3/4"


Clearance
     Wheel Cyl
          .001" to .005"
     Main Cyl.
          .002"	to .004"
	

Wheel Cyl.
     Size
          Front----- 1 1/8" 	
          Rear------ 1

Main Cyl.
     Size
          Front----- 1 	

Troubles and Remedies

BRAKE SYSTEM

Symptom and Probable Cause
Probable Remedy


Pedal Spongy

a. Air in brake lines.
a. Bleed brakes.


All Brakes Drag

a. Mineral oil in system.
a. Flush entire brake system and replace all rubber parts.

b. Improper pedal to push rod clearance.
b. Adjust clearance.

c. Compensating port in main cylinder restricted.
c. Overhaul main cylinder.


One Brake Drags

a. Loose or damaged wheel bearings.
a. Adjust or replace wheel bearings.

b. Weak, broken or unhooked brake retractor spring.
b. Replace retractor spring.

c. Brake shoes adjusted too close to brake drum.
c. Correctly adjust brakes.


Excessive Pedal Travel

a. Normal lining wear or improper shoe adjustment.
a. Adjust brakes.

b. Fluid low in main cylinder.
b. Fill main cylinder and bleed brakes.


Brake Pedal Applies Brakes but Pedal Gradually Goes to Floor Board

a. External leaks.
a. Check main cylinder, lines and wheel cylinder for leaks and make necessary repairs.

b. Main cylinder leaks past primary cup.
b. Overhaul main cylinder.


Brakes Uneven

a. Grease on linings.
a. Clean brake mechanism; replace lining and correct cause of grease getting on lining.

b. Tires improperly inflated.
b. Inflate tires to correct pressure.

c. Spring center bolt sheared and spring shifted on axle.
c. Replace center bolt and tighten "U" bolts securely.


Excessive Pedal Pressure Required, Poor Brakes

a. Grease, mud or water on linings.
a. Remove drums-clean and dry linings or replace.

b. Full area of linings not contacting drums.
b. Free up shoe linkage, sand linings or replace shoes.

c. Scored brake drums.
c. Turn drums and install new linings.

Treadle-Vac Power Brakes

General Description

A new Treadle-Vac is in use on 1957 models. This power brake assembly (fig. 31) is a self contained hydraulic and vacuum unit.

This system provides a reduced pedal travel compared to the conventional braking system. About 50% reduced pedal travel brings the height of the pedal down to approximately that of the accelerator pedal permitting the driver to shift his toe from one pedal to the other without lifting his heel from the floor. Lighter pedal pressures are also obtained.

The power brake replaces the conventional brake system master cylinder and requires no external rods or levers exposed to dirt and moisture. The power brake assembly is mounted to the engine side of the firewall and operates from a suspended pedal which is connected directly to the power brake push rod.

In addition the power brake unit is so constructed that in the event of engine failure and consequent loss of vacuum the brakes will function satisfactorily although more effort is required. The vacuum reserve tank maintains a vacuum which will retain vacuum assist for a limited time if the engine stalls. The vacuum line from the intake manifold also has a vacuum check valve which retains vacuum in the brake system during acceleration, hill climbing, or after the engine has stopped. Two external line connections are necessary, one from the intake manifold (vacuum source) to vacuum reservoir, the other from the power brake into the hydraulic brake system.

The hydraulic reaction type Treadle-Vac consists of four basic components combined into a single assembly (fig. 31).

(a) Vacuum power cylinder.
(b) Power piston, valve, and operating rod assembly.
(c) Hydraulic plunger and reaction rod.
(d) Hydraulic cylinder and reservoir assembly.

The vacuum power piston and the components which make up the valve assembly are connected to the brake pedal through the valve operating rod. The valve operating rod is connected to the valve plunger which operates within the power piston. The valve return spring is incorporated to return the valve plunger and valve operating rod to the released position when the brakes are released.

The valve portion of the Treadle-Vac consists of a poppet valve, an atmospheric port and a vacuum port. The atmospheric port seat is located on the valve plunger while the vacuum port seat is located on the left piston plate.

The valve poppet is assembled in a flexible diaphragm in the power piston. A plate is used in conjunction with the diaphragm to limit the effective area of the diaphragm. When the power unit is in the released position, the poppet return spring overcomes the force on the poppet as a result of the atmospheric pressure on the left side of the poppet and the vacuum on the right side of the poppet so that the return spring holds the poppet on the vacuum poppet seat. A boot type seal is used to seal the opening between the piston plate and the valve operating rod.

When pressure is applied to the power piston, the hydraulic plunger assembly transmits the pressure to the hydraulic cylinder. A reaction rod located within the hydraulic plunger, transmits a "reaction force" back through the rod and counter reaction spring against the valve plunger. The hydraulic cylinder with its fluid reservoir is attached to end of the vacuum cylinder.

The hydraulic cylinder is sealed off from the vacuum cylinder at the hydraulic plunger by a wiper leather seal and a rubber cup type seal.

The reservoir is sealed off from the hydraulic cylinder by the compensating valve at all times except when the Treadle-Vac is in its fully released position. A residual pressure check valve located at the output end of the hydraulic cylinder maintains a slight pressure in the hydraulic brake lines during release of the brakes to prevent the entrance of air into the hydraulic brake lines as in the conventional brake system.

Operation

Released Position
Brake Application
Holding Position and Releasing
No Power Condition

Released Position

With the engine running and the brakes released (fig. 32), vacuum from the engine intake manifold is transmitted to the Treadle-Vac through the check valve, vacuum tubing and reservoir. In the released position, (no pressure applied to brake pedal), the valve operating rod and plunger are held to the left in the power piston by the valve return spring to close the vacuum port and open the atmospheric port. With atmosphere present on both sides of the piston, the unit is in released position. The piston return spring attached to the hydraulic plunger holds both the vacuum piston and hydraulic plunger in the released position. With the hydraulic plunger in the released position the compensating valve is tilted by -the washer near the end of the hydraulic plunger to open the passage between the reservoir and the hydraulic cylinder.

Brake Applications

Applying

As the brakes are applied by the driver (fig. 33), the valve operating rod moves the valve plunger to the right to close the atmospheric port. Further movement of the valve plunger opens the vacuum port. With the vacuum poppet open, the right side of the vacuum piston is also open to vacuum through the passages in the piston. With a partial vacuum on the right side of the piston and atmosphere on the left side of the piston, the differential in pressure creates a force which moves the vacuum piston with the hydraulic plunger to the right. The initial movement of the hydraulic plunger allows the compensating valve to seat and close off the passage between the fluid reservoir and hydraulic piston. As pressure is developed within the hydraulic cylinder, fluid is forced through the residual check valve and brake lines to the wheel cylinders. This same pressure acting against the rubber membrane at the end of the hydraulic plunger transmits a force through the reaction rod and counter reaction spring to the left against the valve plunger which tends to close the vacuum port and bring the power piston to rest. Since the reaction force is in proportion to the hydraulic pressure developed with the hydraulic cylinder and wheel cylinders, it gives the driver a "feel" of the amount of braking.

Holding position and Releasing

Holding

When pressure applied by the driver is held constant (fig. 34), the valve returns to the lap or holding position. In the lap or holding position both the vacuum and atmospheric ports of the poppet valve are closed and no further movement of the power piston will occur unless the force against the brake pedal is increased or decreased.

Releasing

When the driver releases the force applied to the brake pedal, the valve operating rod and valve plunger return to the released position. The vacuum port is then closed and the atmospheric port is opened to admit atmosphere to the right side of the power piston to balance the piston in atmosphere. The power piston return spring is then free to return the power piston and hydraulic plunger to the released position. As the hydraulic plunger approaches the released end of its stroke, the washer on the hydraulic plunger contacts the compensating valve stem to reopen the compensating valve port to allow any excess of fluid to return to the reservoir or fluid to enter the hydraulic cylinder from the reservoir to compensate for any loss of fluid from the brake system.

No Power Condition

If it should be necessary to use the brakes at any time when the engine is not running and there is no reserve vacuum in the system, the brakes can be applied manually, however, more physical effort will be required than when vacuum is present in the system. In the case of "no power" the force applied by the driver to the end of the valve operating rod will be transmitted directly to the hydraulic plunger and to the wheel cylinders.

Care, Maintenance and Adjustments

Removal
Replacement
Inspection
Lubrication
System Tests
Bleeding Instructions
Air Cleaner Service

Removal

1. On the engine side of the dash panel remove vacuum lines from vacuum inlet tube on intake manifold top of engine on V-8 and from intake manifold left side center on 6 cylinder.
2. Remove vacuum line to vacuum reservoir.
3. Remove hydraulic line from hydraulic cylinder.
4. From inside body remove the lever to push rod retaining bolt washer, lock washer and nut and disengage the push rod from the push rod lever.
5. Remove the four bolts and lock washers retaining the power brake unit to the dash panel mounting plate and remove the unit from the vehicle.

1. Insert the push rod through dash panel into body then attach the power brake unit to the mounting plate with four retaining bolts and lock washers.
2. Connect the vacuum line from intake manifold and vacuum line from the vacuum tank to inlet tube on vacuum cylinder.
3. Attach brake hydraulic line.
4. Inside body, connect push rod to lever with retaining bolt washer, lock washer and nut.
5. Bleed the hydraulic system as outlined in Sec. 5.

1. Check the vacuum lines and connection at the engine intake manifold and check valve for possible vacuum loss.
2. Inspect the hydraulic lines and connections at the wheel cylinders for possible hydraulic leaks.
3. Check brakes for scored drums, grease, or brake fluid on linings, worn or glazed linings, and make necessary brake adjustments.
4. Check the vacuum lines and connections at the vacuum inlet tube assembly, check vacuum attachment at vacuum reservoir.
5. Inspect the hydraulic lines and connection attached to the power brake hydraulic cylinder output port for hydraulic leaks.
6. Check the brake fluid level in the hydraulic reservoir. The reservoir should be filled to within 1/2" of the top of the filler cap opening. Inspect the reservoir cover for fluid leaks at gasket.
7. Check air cleaner on brake cylinder and replace hair filter if necessary.
8. Inspect the air cleaner and vacuum inlet tube assembly for loose screws. Check for loose unit mounting bolts.
9. Check brake pedal for binding and misalignment between pedal and push rod.

The Treadle-Vac power brake system is lubricated at the time of assembly and needs no further lubrication.

System Tests

Road test brakes by making a brake application at approximately 20 M.P.H. to determine if braking is even and quick. If pedal has a spongy feel when applying the brake, air is present in the hydraulic system. Bleed the brake system as described in See. 5-8.

With the engine stopped and the transmission in neutral apply the brake several times to destroy all vacuum in the system. Then depress the brake pedal and while holding the brake pedal down, start the engine. If vacuum system is operating, pedal will tend to fall away under foot pressure and less pressure is required to hold pedal in the applied position. If no action is felt, vacuum system is not functioning.

Stop engine and again destroy all vacuum in the system by applying the brakes several times. The depress the brake pedal and hold foot pressure on pedal. If pedal gradually falls away from foot pressure, the hydraulic system is leaking. If the brake pedal travels to within one inch of the toe board, brake shoes require adjustment or relining.

Bleeding Instructions

The power brake system may be bled manually or with a pressure bleeder as outlined in Sec. 5. Use only G.M. Super 11 or its equivalent. Do not use the vacuum assist while bleeding. The engine should not be running and the vacuum reserve should be reduced to zero by applying the brake several times with the engine off before starting the fluid bleeding procedure.

Air Cleaner Service

The air cleaner used with power brake should be cleaned at least twice a year. To clean, remove air cleaner and wash thoroughly in cleaning solvent and allow to dry before reinstalling.

Major Service Operations

Disassembly
Cleaning
Inspection
Assembly

Disassembly

1. Hold Treadle-Vac in vise (fig. 35) and remove rubber dust guard (1) and felt (2) from valve push rod. Bend out tabs (3) on end plate and remove end plate (4) and gasket (5). Slide vacuum hose (6) off vacuum tube attached to vacuum cylinder (7). Remove air cleaner attaching screw (12) and separate gasket (11); remove shell (10), hair (9) and rubber seal (8).
2. Remove tube and plate attaching screws (8), (fig. 36), tube and plate (9) and gasket (10). Remove burrs from inside of vacuum cylinder at air cleaner and vacuum tube attaching screw holes. Pull out vacuum piston and valve assembly (6) from vacuum cylinder (1). Remove counter reaction spring (5) and washer (4) from end of hydraulic plunger (2). Then remove "O" ring seal (3) from groove in hydraulic plunger.
3. Push in on spring retainer plate (2) (fig. 37), sufficient to release "C" washer (1). While holding spring compressed, slide "C" washer out of groove in hydraulic plunger (4). Remove retainer plate (2) and vacuum piston return spring (3).
4. Scribe across vacuum cylinder (2) ( fig. 38), and hydraulic master cylinder (7). Hold hydraulic master cylinder in vise and remove three vacuum cylinder attaching screws and lockwashers (1) (use 7/16" socket with extension). Lift off vacuum cylinder, remove Vellumoid gasket (3) and rubber ring gasket (4). Push hydraulic plunger (6) into hydraulic master cylinder and remove leather seal (5) from master cylinder flange recess.
5. Scribe across master cylinder and cover (3) (fig. 39). Remove filler plug (2), then remove six cover attaching screws (1), cover (3) and gasket (4). Remove compensating valve (5) (use 1 1/8" thin wall socket). Loosen outlet fitting (6) with 1 1/4" wrench. DO NOT REMOVE OUTLET FITTING AT THIS TIME. Remove retainer ring (7). When retainer ring is of the Truarc type, use Truarc Pliers. When retainer ring is of Spirolex type, use ice pick or other sharp tool to pry ring out of groove. Pull hydraulic plunger (12) out of hydraulic master cylinder and remove steel washer (8), fiber guide washer (9), rubber cup (10) and cup retainer (11) from hydraulic plunger. Remove membrane seal (15) and Teflon washer (14) from hydraulic plunger (13), see inset upper left.
6. Remove hydraulic master cylinder from vise (fig. 40) and then remove outlet fitting (1), residual check valve spring (4), valve cup and retainer (3) from master cylinder (5). Remove gasket (2) from fitting (1).
7. Clamp compensating valve fitting (3) in vise (fig. 41); spread and remove spring retainer (1), then remove spring (2), valve stem and poppet (5) and fitting gasket (4) from compensating valve fitting.
8. Remove valve rod seal (fig. 42) and stop (17) from piston (20) and valve operating rod (19). Remove vacuum hose (23) from tube on rear piston plate. Remove felt retainer ring (18), felt (20) and expander spring (21) from rear piston plate. Turn piston over, clamp rear piston plate (15) in vise and remove four cap screws (1), piston plate (2), diaphragm plate (5) and return spring (6). Drive out fiber washer (4) and remove rubber bumper (3) from front piston plate (2).Remove diaphragm parts group (7) thru (10)as an assembly and then separate these parts. Lift off leather piston packing (11) and return spring (12). Remove valve plunger (14) with valve operating rod (16) and remove rubber bumper (13) from end of valve plunger.

NOTE: On those units where diameter of valve operating rod is larger than hole in piston, hold assembly with valve plunger (14) down and inject alcohol in valve plunger opening around rod to wet the rubber lock. Then drive valve plunger off valve operating rod.

Thoroughly wash all parts in alcohol or a commercial cleaning fluid. Use only alcohol on rubber parts or parts containing rubber. Use air hose to blow dirt and cleaning fluid out of recesses and internal passages. If bore of vacuum cylinder has signs of rust or corrosion, it can be removed with fine emery cloth.

NOTE: It is important that all cleaned parts be placed on clean paper or cloth to prevent the possibility of dirt being assembled into the power brake.

Inspection

In addition to the parts contained in the power brake repair kits, inspection of the remaining parts should be made as directed below and parts replaced as necessary.

Vacuum Cylinder Shell

Inspect shell for scoring, pitting, dents or nicks, or damaged threads in nuts or mounting flanges.

Hydraulic Cylinder Casting

Examine the bore down one inch from the open end. For the hydraulic cup to seal properly, this portion of the bore must be free from scores, deep scratches and corrosion. The sealing surfaces at the reservoir cover, compensating port and hydraulic outlet port must be free of scoring, pitting, dents and nicked edges. Also check casting for cracks and damaged threads.

Hydraulic Port Fitting

The surface at the small end of the fitting must be free of scoring or corrosion which might prevent sealing with the rubber cup of the residual check valve.

Compensating Port Fitting

Inspect surface around port opening, inside threaded end of fitting, for scoring or corrosion which might prevent the fitting sealing properly with the compensating valve.

Vacuum Supply Tube

Make sure the tube braze is secure and the tube plate is not distorted. Piston Plates Examine plates for cracks and damaged threads. Inspect plunger bore in rear piston plate and poppet seats for scratches and nicks. Do not attempt to refinish bore. Replace with new plate if necessary.

Hydraulic Plunger and Washer Assembly

Inspect polished surface for scoring, pitting or dents. Do not attempt to refinish plunger surface -replace with new assembly if necessary.

Push Rod and Plunger Assembly

Rod must pivot freely in plunger without any noticeable end play. Inspect plunger for scoring, pitting or dents on outside diameter polished surfaces. Do not attempt to refinish plunger surface replace with new assembly if necessary.

Vacuum Cylinder End Plate

Examine end plate for distortion.

Assembly

Before assembly dip all hydraulic system parts in clean alcohol and place in a clean pan or on a clean paper.

1. Insert grooved end of compensating valve poppet (fig. 43) and stem (5) through hole in fitting (3) from threaded end. Assemble large diameter end of spring (2) over stem, hold valve poppet on seat, compress spring and assemble retainer washer (1) in groove of valve stem. Squeeze ends of washer together with pliers. Assemble new gasket (4) over threads of compensating valve fitting.
2. Assemble new gasket (2) (fig. 44) over threads of hydraulic outlet fitting (1). Hold outlet fitting in vertical position and insert cone end of cup and retainer (3) in fitting. Place check valve spring (4) in recess of retainer. Hold hydraulic master cylinder upside down as shown and thread outlet fitting (1) into hydraulic master cylinder hand tight.
3. Hold master cylinder in vise (fig. 45). Assemble Teflon washer (14) and rubber membrane (15) at end of hydraulic plunger (13), see inset upper left. Insert membrane end of plunger (12) in cylinder and assemble seal parts over end of plunger as follows: cup retainer (11), with counter-bored side away from washer on plunger. Dip cup (10) in brake fluid before assembly, assemble fiber guide washer (9) and steel spacer washer (8). Slide seal parts into recess of cylinder, push plunger into end of its stroke and assemble retainer ring (7) in ring groove (use Tool 5403). Assemble compensating valve (5) in threaded hole of cylinder and securely tighten (use 1 1/8" thin wall socket). Place new cover gasket (4) on master cylinder, align cover (3) to scribe marks, replace and securely tighten cover screws (1). Place new gasket on filler cap (2) and assemble filler cap. Securely tighten outlet fitting (6).
4. Pull out hydraulic plunger (6) (fig. 46), place Tool 5405 (8) over end of plunger and assemble leather seal (5) over seal tool with hp of leather toward hydraulic master cylinder, see inset upper left. Press seal into recess of hydraulic master cylinder and then remove seal assembly tool. Place gasket (4) in recess of master cylinder. Insert three cap screws (1) through holes in end of vacuum cylinder (2) and holes in gasket (3). Align vacuum cylinder and hydraulic master cylinder with holes for air cleaner on same side as reservoir cover or to scribe marks, assemble three cap screws and securely tighten screws. Wipe out inside of vacuum cylinder and then pull out hydraulic plunger to end of its stroke.
5. Place larger diameter end (fig. 47) of vacuum piston return spring (3) in vacuum cylinder and assemble retainer plate (2) over end of hydraulic plunger (4) as shown. Compress spring and assemble "C" washer (1) in second groove of hydraulic plunger.
6. If valve operating rod (16) and valve plunger (14) are separated (fig. 48), assemble valve operating rod to valve plunger as follows: dip valve plunger in alcohol before inserting valve rod; make certain ball end of rod is locked in place in the valve plunger. It may be necessary to tap end of valve operating rod to seat ball end in valve plunger. Insert valve operating rod through hole in rear piston plate, assemble bumper (13) in end of valve plunger and assemble spring (12) over end of plunger. Assemble leather packing (11) on piston with lip side toward valve operating rod. Assemble round diaphragm plate (9) in groove of diaphragm (8) and then assemble diaphragm over shoulder of valve poppet (10). Place this assembly in recess of piston plate and assemble spring retainer (7) and diaphragm plate (5) in diaphragm. Align holes. Assemble poppet return spring (6) over flange of spring retainer.
   • Remove burrs from center bore of front piston plate (2) and assemble rubber bumper (3) fiber washer (4) in recess of plate from side shown. Stake in place at three points. Place front piston plate on rear piston plate making certain by-pass holes are in register and assemble four attaching cap screws (1). Leave screws loose.
   • Place Tool 5406 over piston leather, turn piston assembly upside down and assemble expander spring (21) against inside lip of leather packing as shown. Dip felt (20) in Bendix Vacuum Cylinder Oil and assemble against expander spring. Assemble retainer plate (18) as shown making certain plate is securely anchored in grooves at four projections of piston assembly. Securely tighten four screws. Dip valve rod seal and bumper (17) in brake fluid and assemble over end of valve operating rod (19) making certain seal is seated in groove of piston assembly.
7. Apply thin film of vacuum cylinder oil (fig. 49) to inside of cylinder (1). Assemble "O" ring seal (3) in outer groove of hydraulic plunger (2); then assemble washer (4) and counter reaction spring (5) in counter bore of plunger. Assemble hose (7) to piston as shown, insert piston in cylinder with free end of hose in line with center of elongated hole (11). Place new gasket (10) on tube and plate (9), slide hose on tube approximately %" and attach tube and plate with screws (8). Operate piston through its full stroke several times to make certain hose does not rub against cylinder or piston. Reposition hose when necessary.
8. Assemble rubber seal (8) on edges of air cleaner shell (10) (fig. 50). Attach air cleaner to vacuum cylinder using screw (12) and gasket (11). With 6" scale or other similar tool, push hair (9) into open spaces at each end of air cleaner. Align holes on end plate (4) and gasket (5) with holes in flange of cylinder and bend over two tabs on end plate to secure end plate and gasket to cylinder. Assemble valve rod felt (2) in fold "A" of rubber guard (1). Dip small end of guard in brake fluid and assemble guard and felt over valve rod. Attach hp of rubber guard over "scallops" at center of end plate.

Brake troubles may be easily diagnosed if the complaint is understood. The trouble will always show up in one or more of the four ways listed below. Related parts of the power brake system should be checked before dismantling the power brake when a malfunctioning brake system is experienced.

1. Hard pedal feel caused by-
• Glazed linings
• Grease or brake fluid on the linings
• Bound up brake pedal linkage
• Sticking vacuum check valve
• Collapsed vacuum hose
• Plugged vacuum fittings
• Leaking vacuum reserve tank
• Internal vacuum hose loose or restricted
• Jammed vacuum cylinder piston
• Vacuum leaks in unit caused by loose piston plate screws
• Faulty diaphragm rubber stop in reaction diaphragm
• Faulty vacuum cylinder piston seal
2. Severe brakes may be caused by-
   • Grease or brake fluid on linings
   • Scored drums
   • Reaction diaphragm leakage
   • Broken counter-reaction spring
   • Restricted diaphragm passage
   • Sticking vacuum action (Do Not Oil)
3. Pedal goes to floor for almost to floor), caused by-
   • Brakes require adjustment
   • Air in hydraulic system
   • Hydraulic leak in lines or at wheel cylinders
   • Fluid reservoir needs replenishing
   • Compensating valve leak
   • Hydraulic piston seal leak
   • Compensating port or outlet fitting seal leak
4. Brakes fail to release (or slow release) due to-
   • Brakes improperly adjusted
   • Bound up brake pedal linkage
   • Restricted air cleaner or passages
   • Excessive hydraulic seal friction
   • Compensator port plugged
   • Faulty residual check valve
   • Piston stroke interference
   • Sticky vacuum valve (Do Not Oil)
   • Broken piston return spring
   • Dry vacuum piston leather packing

General Description

The Hydrovac is a vacuum-hydraulic unit for power braking which utilizes engine intake manifold vacuum and atmospheric pressure for its operation. The Hydrovac adds to the pressure created physically in the master cylinder and delivers this combined pressure to the wheel cylinders. It is a self contained unit having no external rods or levers exposed to dirt and moisture. Construction of the Hydrovac is such that in case of engine failure and loss of vacuum, the brakes will function as in a conventional brake system. The Hydrovac is connected in the brake system between the master cylinder and the wheel cylinders. A vacuum line connection is made from the Hydrovac to the engine intake manifold through a vacuum check valve. The air cleaner is built integral with the control valve.

This Hydrovac braking system is used as a regular production option on all models equipped with standard transmission. In this application the brake pedal height is the same as clutch pedal height.

Internally, the Hydrovac is composed of three major sections; (See Figure 51).

1. Vacuum power cylinder.
2. Hydraulic actuating cylinder.
3. Hydraulically actuated vacuum control valve. The vacuum power cylinder consists of: a cylinder shell (1) clamped to end plate (5) by four bolts (15). The cylinder shell contains power piston (2), piston return spring (14), and push rod (13). Control tube (3) connects the chamber to the left of the power cylinder to the right side of diaphragm assembly (8) of the control valve. The chamber to the right of the power cylinder piston is connected through a vacuum outlet (16) to the intake manifold. Through a passage in the end plate this chamber is also connected to the left side of diaphragm assembly (8).