AMC Coupes/Sedans/Wagons 1975-1988 Repair Information

Master Cylinder



See Figures 1 through 5

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Fig. Fig. 1: Exploded view of a common master cylinders components

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Fig. Fig. 2: Use a line wrench to loosen the fuel line-to-master cylinder nuts

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Fig. Fig. 3: Disengage the fuel lines from the master cylinder

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Fig. Fig. 4: Unfasten the master cylinder-to-firewall retaining bolts

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Fig. Fig. 5: Remove the master cylinder from the vehicle

  1. Detach the front and rear brake lines from the master cylinder. On cars with drum brakes, the check valves will keep the fluid from draining our of the cylinder. If the car has disc brakes, one of both of the outlets must be plugged, to prevent fluid loss.
  3. Remove the nuts which attach the master cylinder to the firewall of the power brake booster.
  5. On cars that don't have power brakes, detach the pedal pushrod from the brake pedal.
  7. Remove the master cylinder from the car.
  9. Installation is the reverse of removal. Bleed the brake system once the master cylinder has been installed.


If the master cylinder leaks externally, or if the pedal sinks while being held down, the master cylinder is worn. There are three possible solutions:

  1. Buy a new master cylinder.
  3. Trade the old one in on a rebuilt unit.
  5. Rebuild the old one with a rebuilding kit.

  1. Remove the cylinder from the car and drain the brake fluid.
  3. Mount the cylinder in a vise so that the outlets are up and remove the seal from the hub.
  5. Remove the stop screw from the bottom of the front reservoir.
  7. Remove the snapring from the front of the bore and remove the primary piston assembly.
  9. Remove the secondary piston assembly using compressed air or a piece of wire. Cover the bore opening with a cloth to prevent damage to the piston.
  11. Clean the metal parts in brake fluid and discard all rubber parts.
  13. Inspect the bore for damage or wear, and check the pistons for damage and proper clearance in the bore.
  15. If the bore is only slightly scored or pitted it may be honed. Always use hones that are in good condition and completely clean the cylinder with brake fluid when honing is completed. If there is any evidence of contamination in the master cylinder, the entire hydraulic system should be flushed and refilled with clean brake fluid. Blow out all passages with compressed air.
  17. Install new secondary seals in the two grooves in the flat end of the front piston. The lips of the seals will be facing away front each other.
  19. Install a new primary seal and the seal protector on the opposite end of the front piston with the lips of the seal facing outward.
  21. Coat the seals with brake fluid. Install the spring on the front piston with the spring retainer in the primary seal.
  23. Insert the piston assembly, spring end first, into the bore and use a wooden rod to seat it.
  25. Coat the rear piston seals with brake fluid and install them into the piston grooves with the lips facing the spring end.
  27. Assemble the spring onto the piston and install the assembly into the bore spring first. Install the snapring.
  29. Hold the piston assembly at the bottom of the bore and install the stop screw. Install a new seal on the hub. Bleed the cylinder as shown, before installation. Install the cylinder on the car. Bleed the system of air.

Clean, high quality brake fluid is essential to the safe and proper operation of the brake system. You should always buy the highest quality brake fluid that is available. If the brake fluid becomes contaminated, drain and flush the system, then refill the master cylinder with new fluid. Never reuse any brake fluid. Any brake fluid that is removed from the system should be discarded.


See Figures 6 and 7

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Fig. Fig. 6: Bleeding the front disc brakes

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Fig. Fig. 7: Loosen the bleeder screw, press the brake pedal to the floor, then close the bleeder screw

The purpose of bleeding brakes is to expel air trapped in the hydraulic system. The system must be bled whenever the pedal feels spongy, indicating that compressible air has entered the system. It must also be bled whenever the system has been opened or leaking. You will need a helper for this job.

When bleeding brakes the stem on the front of the brake combination valve must be held out 0.060 in. (1.52mm).

  1. Clean the bleed screw at each wheel.
  3. Attach a small rubber hose to one of the bleed screws and place the end in a container of brake fluid.
  5. Fill the master cylinder with brake fluid. Check the level often during bleeding. Pump up the brake pedal and hold it.
  7. Open the bleed screw about 1 / 4 turn, press the brake pedal to the floor, close the bleed screw, and slowly release the pedal. Continue until no more air bubbles are forced from the cylinder on application of the brake pedal.
  9. Repeat the procedure on the remaining wheel cylinders.

Disc brakes may be bled in the same manner as drum brakes, except that:

  1. It usually requires a longer time to bleed a disc brake thoroughly.
  3. The disc should be rotated to make sure that the piston has returned to the unapplied position when bleeding is completed and the bleed screw closed.


See Figures 8 and 9

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Fig. Fig. 8: Typical combination valve componentsexcept Pacer models

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Fig. Fig. 9: Cross-sectional view of a pressure differential valvePacer models

The warning light on the dashboard is activated by a differential pressure switch located below the master cylinder. The signal indicates a loss of fluid pressure in either the front or rear brakes, and warns the driver that a hydraulic failure has occurred.

The pressure differential warning valve is a housing with the brake warning light switch mounted centrally on top. Directly below the switch is a bore containing a piston assembly. The piston assembly is located in the center of the bore and kept in that position by equal fluid pressure on either side. Fluid pressure is provided by two brake lines, one coming from the rear brake system and one from the front brakes. If a leak develops in either system (front or rear), fluid pressure to that side of the piston will decrease or stop causing the piston to move in that direction. The plunger on the end of the switch engages with the piston. When the piston moves off center, the plunger moves and triggers the switch to activate the warning light on the dash.

After repairing and bleeding any part of the hydraulic system the warning light may remain on due to the pressure differential valve remaining in the off-center position. All models have a self-centering valve. After repairs or bleeding have been performed, center the valve by applying moderate pressure on the brake pedal. This will turn on the light.

Front wheel balancing, on the car, of cars equipped with disc brakes may also cause a pressure differential in the front branch of the system.