GM Firebird 1967-1981 Repair Guide

Master Cylinder

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REMOVAL & INSTALLATION



See Figures 1 through 7

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Fig. Fig. 1: Exploded view of the brake master cylinder mounting 1967-69 models



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Fig. Fig. 2: Exploded view of a common brake master cylinder mounting for newer models with power brakes



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Fig. Fig. 3: To remove the brake master cylinder, first drain the fluid out of the reservoir ...



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Fig. Fig. 4: ... then loosen the fittings ...



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Fig. Fig. 5: ... and detach the brake lines from the master cylinder



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Fig. Fig. 6: Remove the two mounting nuts ...



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Fig. Fig. 7: ... and lift the master cylinder away from the power booster


WARNING
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.

  1. Disconnect and plug the hydraulic line(s) at the master cylinder.
  2.  
  3. Remove the two retaining nuts and lockwashers which hold the cylinder to the firewall.
  4.  

On non-power brakes, disconnect the pushrod at the brake pedal.

  1. Remove the master cylinder, gasket, and rubber boot.
  2.  

To install:
  1. Position the master cylinder on the firewall, making sure that the pushrod goes through the rubber boot into the piston.
  2.  

On non-power brakes, reconnect the pushrod clevis to the brake pedal. If so equipped, thread the jam nut down to the pushrod shoulder, thread the clevis down to the jam nut, and tighten the nut (against the clevis) to 14 ft. lbs.

  1. Install the nuts and lockwashers.
  2.  
  3. Install the hydraulic line(s), then check brake pedal free-play.
  4.  
  5. Bleed the brakes as described later in this section.
  6.  

Cars having disc brakes do not have a check valve in the front outlet port of the master cylinder. If one is installed, front discs will immediately wear out due to residual hydraulic pressure holding the pads against the rotor.

OVERHAUL



See Figure 8

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Fig. Fig. 8: Exploded view of a common brake master cylinder

  1. Remove the master cylinder from the car.
  2.  
  3. Remove the mounting gasket and boot, and the main cover. Empty the cylinder of all fluid.
  4.  
  5. Place the cylinder in a padded vise and remove the pushrod retainer and the secondary piston stop bolt that are found inside the front reservoir.
  6.  
  7. Remove the retaining ring and primary piston assembly.
  8.  
  9. Direct compressed air into the piston stop screw hole to force the secondary piston, spring, and retainer from the cylinder bore. If compressed air isn't available, use a hooked wire to pull out the secondary piston.
  10.  
  11. Check the brass tube fitting inserts and, if damaged, remove them; if not, leave them in place.
  12.  
  13. If insert replacement is necessary, thread a no. 6 5 / 8 in. self-tapping screw into the insert. Hook the end of the screw with a claw hammer and pull out the insert.
  14.  
  15. An alternative (but more troublesome) way to remove the inserts is to drill out the outlet holes with a 13 / 64 in. drill and thread them with a 1 / 4 in. 20 tap. Position a thick washer over the hole to serve as a spacer and thread a 1 / 4 in. #20 3 / 4 in. hex-head bolt into the insert and tighten the bolt until the insert is free.
  16.  
  17. Use only [cf2]denatured alcohol or brake fluid and compressed air to clean the parts. Slight rust may be removed with crocus cloth.
  18.  


CAUTION
Never use any mineral-based solvents (gasoline, kerosene, etc.) for cleaning. It will quickly deteriorate rubber parts.

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 the assembled piece in the overhaul kit.
  10.  
  11. Coat the cylinder bore and the secondary piston's inner and outer seals with brake fluid. Assemble the secondary piston spring to its retainer and place them over the end of the primary seal.
  12.  
  13. Insert the combined spring and piston assembly into the cylinder and, using a pencil, seat the spring against the end of the bore.
  14.  
  15. Coat the primary piston seals with brake fluid and push it (pushrod receptacle end out) into the cylinder.
  16.  
  17. Hold the piston in and snap the retaining ring into place.
  18.  
  19. Continue to hold the piston down to make sure that all components are seated and insert the secondary piston stop screw in its hold in the bottom of the front reservoir. Tighten the screw to 25-40 in. lbs.
  20.  
  21. Install the reservoir diaphragm and cover.
  22.  
  23. It will save time to bleed the cylinder before installing it in the car. Do so in the following manner:
    1. Install plugs in the outlet ports.
    2.  
    3. Place the unit in a vise with the front end tilted slightly downward. Do not overtighten the vise.
    4.  
    5. Fill both reservoirs with clean fluid.
    6.  
    7. Using a smooth, round rod, push in on the primary piston.
    8.  
    9. Release the pressure on the rod and watch for air bubbles in the fluid. Keep repeating this until the bubbles disappear.
    10.  
    11. Loosen the vise and position the cylinder so that the front end is tilted slightly upward. Repeat Steps d and e.
    12.  
    13. Place the diaphragm and cover on the reservoir.
    14.  

  24.  

Master cylinder overhaul on cars with power brakes is the same.

 
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