GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Master Cylinder

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REMOVAL & INSTALLATION



See Figures 1, 2, 3, 4, 5 and 6

Vacuum Power Brakes

On vacuum power brake equipped models, the master cylinder can be removed without removing the power vacuum cylinder from the vehicle.


WARNING
Only use flare nut wrenches when removing the brake lines from any component. Standard wrenches will damage the flare nut and line.

  1. Clean the area around the master cylinder.
  2.  
  3. Disconnect the hydraulic lines at the master cylinder using flare nut wrenches only. Plug or tape the ends of the lines to prevent dirt from entering and to prevent fluid from leaking out.
  4.  
  5. Remove the master cylinder attaching nuts and remove the master cylinder.
  6.  
  7. Drain the master cylinder.
  8.  


CAUTION
Be careful to keep brake fluid away from all body paint. The fluid acts like paint remover, and a few drops will quickly bubble any paint with which it comes in contact.



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Fig. Fig. 1: Master cylinder mounting on cars equipped with vacuum power brakes



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Fig. Fig. 2: Use a flare nut wrench when removing the brake lines



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Fig. Fig. 3: Use a pan to catch any brake fluid; be sure to tape or plug the lines after removing them from the master cylinder



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Fig. Fig. 4: The master cylinder attaching nuts can be removed after the lines have been disconnected



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

Powermaster® Power Brakes

This system uses an electrically operated hydraulic pump and an accumulator (pressure storage system) to provide power braking assist.


CAUTION
Because of the very high pressures used, always follow procedures very carefully, being especially sure to discharge the system prior to disconnecting anything.

  1. Disconnect the negative (-) battery cable. Turn the ignition switch off. Then, discharge all pressure from the system by applying the brake pedal with maximum force (50 lbs. or more) at least 10 full strokes.
  2.  
  3. Disconnect the electrical connector from the pressure switch, located at the rear of the master cylinder on top.
  4.  
  5. Disconnect the electrical connector from the front of the pump, located under the master cylinder.
  6.  
  7. Disconnect the brake and hydraulic pressure pipes at the master cylinder and tape or cap the openings. Use a flare nut wrench only.
  8.  
  9. Remove the two attaching nuts. Pull the clevis pin out of the linkage to the brake pedal. Remove the unit.
  10.  

To install:
  1. Position the unit onto the firewall and torque the mounting nuts to 22-30 ft. lbs. (30-41 Nm) and the brake pipe nuts to 10-13 ft. lbs. (14-17 Nm).
  2.  
  3. Connect the brake and hydraulic pressure pipes at the master cylinder using a flare nut wrench.
  4.  
  5. Connect the electrical connector to the front of the pump, located under the master cylinder.
  6.  
  7. Connect the electrical connector to the pressure switch, located at the rear of the master cylinder on top.
  8.  
  9. Bleed the system as described in the System Bleeding procedures in this section.
  10.  



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Fig. Fig. 6: Powermaster® master cylinder mounting

OVERHAUL



See Figures 7, 8, 9, 10, 11, 12, 13 and 14

Except Powermaster®

The models covered in this guide are equipped with either Moraine or Bendix master cylinders. The rebuilding kits may differ slightly, but the procedures are the same. Follow the instructions that come with each particular kit.

Overhaul procedures for power brake master cylinders and manual brake master cylinders is the same. Note the procedure below for Powermaster® units.

  1. Remove the master cylinder from the vehicle.
  2.  
  3. Remove the mounting gasket and boot, and the main cover, and purge the unit of its fluid.
  4.  
  5. Secure the cylinder in a vise and remove the pushrod retainer and secondary piston stop bolt found inside the forward reservoir (Moraine iron cylinder only).
  6.  

The plastic composite master cylinder reservoir can be removed by prying against the cylinder and reservoir. Work the reservoir from the rubber grommets using care not to damage either component.

  1. Compress the retaining (lock) ring and extract it along with the primary piston assembly from the end of the bore.
  2.  
  3. Blow compressed air into the piston stop screw hole, if equipped, to force the secondary piston, spring, and retainer from the bore of the cylinder. An alternative method is to use hooked wire to snag and extract the secondary piston.
  4.  
  5. Check the bass tube fitting inserts and if they are damaged, remove them. Leave undamaged inserts in place.
  6.  
  7. If replacement is necessary, thread a 3 / 16 in. × 5 / 8 in. self-tapping screw into the insert. Hook the end of the screw with a claw hammer and pry the insert free.
  8.  
  9. An alternative way to remove the inserts is to first drill the outlet holes to 13 / 64 in. and thread them with a 1 / 4 in.-20 tap. Position a thick washer over the hole to serve as a spacer, and then thread a 1 / 4 in.-20 × 3 / 4 in. hex head bolt into the insert and tighten the bolt until the insert is freed.
  10.  
  11. Use denatured alcohol and compressed air to clean the parts. Slight rust may be removed with crocus cloth. Never use petroleum-based solvents to clean brake parts. Also, a brake hone is helpful in removing varnish and slight rust.
  12.  

To assemble:
  1. Replace the brass tube inserts by positioning them in their holes and threading a brake line tube nut into the outlet hole. Turn down the nut until the insert is seated.
  2.  
  3. Check the piston assemblies for correct identification and, when satisfied, position the replacement secondary seals in the twin grooves of the secondary piston.
  4.  
  5. The outside seal is correctly placed when its lips face the flat end of the piston.
  6.  
  7. Slip the primary seal and its protector over the end of the secondary piston opposite the secondary seals. The flat side of this seal should face the piston's compensating hole flange.
  8.  
  9. Replace the primary piston assembly with assembled pieces in the overhaul kit.
  10.  
  11. Moisten the cylinder bore and the secondary piston's inner and outer seals with clean brake fluid. Assemble the secondary piston spring to its retainer and position them over the end of the primary seal.
  12.  
  13. Insert the combined spring and piston assembly into the cylinder and use a small wooden dowel or pencil to seat the spring against the end of the bore.
  14.  
  15. Moisten the primary piston seals with brake fluid and push it, pushrod receptacle end out, into the cylinder.
  16.  
  17. Keep the piston pushed in and snap the retaining (lock) ring into place.
  18.  
  19. Relax the pressure on the pistons and allow them to seek their static positions.
  20.  
  21. Replace the secondary piston stop screw and torque it to 25-40 inch. lbs. (3-4 Nm), if so equipped.
  22.  
  23. Replace the reservoir diaphragm and cover.
  24.  
  25. Install the master cylinder and bleed the entire system.
  26.  



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Fig. Fig. 7: Iron reservoir master cylinder



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Fig. Fig. 8: Removing the plastic master cylinder reservoir



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Fig. Fig. 9: It may be helpful to slightly depress the piston when removing the lock ring



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Fig. Fig. 10: Removing the primary piston from the master cylinder



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Fig. Fig. 11: Removing the secondary piston



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Fig. Fig. 12: Seals can be removed from the piston with a small pick or awl



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Fig. Fig. 13: Internal components of a master cylinder

Powermaster® Units

CAUTION
The Powermaster® master cylinder unit must be overhauled on a clean bench. Be especially careful that there are no traces of ordinary, mineral type lubricants, as these will ruin the seals.

  1. Remove the reservoir cover and diaphragm. Drain all brake fluid.
  2.  
  3. Remove:
    1. The pressure switch and O-ring.
    2.  
    3. Accumulator and O-ring
    4.  
    5. Electro-hydraulic pump and pressure hose assembly
    6.  
    7. The sump hose clamps and the sump hose.
    8.  
    9. The tube and nut assembly that runs from the sump hose to the master cylinder.
    10.  
    11. Remove the two pump brackets from the master cylinder.
    12.  

  4.  


CAUTION
Do not scratch or deform in any way the outside diameter and sealing surface at the pushrod end of the power piston assembly. Also avoid such damage to the bores of the Powermaster® body.

  1. Remove the retainer (it resembles a piston ring) from the groove in the rear of the unit. Then, pull the pushrod to remove the boot, retainer, pushrod and power piston group.
  2.  
  3. Remove the retainer, boot, pushrod, socket group, and piston guide from the power piston assembly.
  4.  
  5. Remove the O-ring from the piston guide. Then, remove the O-rings from the power piston assembly and piston guide.
  6.  
  7. Remove the reaction body group from the power piston assembly.
  8.  
  9. Remove the reaction piston and reaction disc from the reaction body group.
  10.  


CAUTION
Do not disassemble the reaction body group or power piston assembly any further! If either are damaged, they must be replaced only as complete assemblies.

  1. Use compressed air cautiously, building pressure slowly, to remove the primary and secondary piston assemblies. Direct the air pressure into the outlet port at the blind end of the master cylinder body. Block the port at the other end of the body.
  2.  
  3. Remove the secondary seal, spring retainer, and primary seal from the secondary piston.
  4.  
  5. Remove the spring from the master cylinder body bore.
  6.  
  7. Mount the master cylinder in a vise with the outboard side upward, clamping the assembly by the mounting flange located at the rear, and not the body itself . Then, carefully pry the reservoir off the body with a prybar.
  8.  
  9. Remove the reservoir grommets. Then, gently tap an Easy-Out® type threaded remover tool into the bore of the valve seat. Pull the tool straight out and remove the seat and seal. Discard the seat.
  10.  
  11. Remove the poppet and spring and discard the spring.
  12.  

To assemble:
  1. Clean all parts except the pressure switch and electro-hydraulic pump in denatured alcohol. If necessary, wipe external surfaces of the pressure switch and electro-hydraulic pump clean with a cloth dampened slightly in denatured alcohol.
  2.  
  3. Inspect all metal parts for cracks, distortion or other damage. Inspect the primary piston sealing surfaces for scoring, deep scratches, or other damage where the damage could cause leaks. Replace the assembly if any of these defects are found.
  4.  
  5. Inspect the power piston and master cylinder bores for scoring or corrosion. Replace the assembly if either problem is noted.
  6.  


CAUTION
Do not attempt to use an abrasive means to clean up these bores, or dangerous driving conditions could result!

Use clean, fresh brake fluid to lubricate all parts at sliding surfaces prior to assembly. Lubricate O-rings, grommets, and seals with the same fluid (all should be replaced). Lubricate both the master cylinder and power piston bores with the same fluid prior to installing the piston assemblies into these bores.

  1. Install a new spring and poppet into the body of the Powermaster® unit. Then, install a new valve seat and seal. Bottom these out by threading the nut of the nut and tube assembly into the body port.
  2.  
  3. Remove the assembly from the vise. Install the three grommets (of two different sizes) into the top of the body making sure they are fully seated . Then, lay the reservoir down on its upper surface and install the master cylinder onto the reservoir from above, holding it upside down.
  4.  
  5. Install the remaining internal components in reverse order. When installing the retainer for the power piston group, depress the piston guide and power piston. Bench bleed the master cylinder side of the unit by filling the reservoirs and working the pistons back and forth.
  6.  
  7. Install the brackets with the mounting bolts. Install the sump hose, clamps, and hydraulic tube. Install the electro-hydraulic pump and pressure hose and clamp.
  8.  
  9. Install the accumulator and O-ring. Install the pressure switch and O-ring.
  10.  
  11. Install the master cylinder in the vehicle and make all connections as described above.
  12.  
  13. Fill both sides of the reservoir with clean, fresh brake fluid meeting the standards shown on the reservoir cover. Then, turn the ignition switch on. Time the running of the pump with your watch. It must not run more than 20 seconds. Have an assistant shut the ignition switch off after 20 seconds if the pump does not cycle off by itself. With the pump running, the fluid level in the booster side of the reservoir should drop. If necessary, add just enough brake fluid to keep the reservoir pump port covered and ensure an adequate supply of air-free fluid to the pump.
  14.  
  15. When the pump stops, check to make sure fluid does not flow back into the reservoir from the booster and check for leaks from the reservoir.
  16.  
  17. Install the reservoir cover securely. Then, pump the brake pedal fully 10 times. Remove the reservoir cover and fill the reservoir to the full line. Again, turn the ignition switch on and time the operation of the pump. It should not run more than 20 seconds. Make sure fluid remains above the level of the reservoir pump port. Again, install the reservoir cover.
  18.  
  19. Turn the ignition switch on and then apply and release the brake pedal to cycle the pump on and off. Count the cycles and repeat the process until the total reaches 15. Make sure the pump does not run more than 20 seconds each cycle (turn the key off if necessary). Recheck the fluid levels and replenish. Check that the pump does not cycle on and off unless you apply the brakes.
  20.  
  21. Install the assembly onto the vehicle and bleed the entire system.
  22.  



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Fig. Fig. 14: Exploded view of a Powermaster® master cylinder

 
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