Chevrolet Full-size Cars 1968-1978 Repair Guide

Master Cylinder

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REMOVAL & INSTALLATION





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Fig. Fig. 1 Brake master cylinder mounting for vehicles with manual brakes



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Fig. Fig. 2 Brake master cylinder mounting for vehicles with power brakes. Shown is a master cylinder with a detachable reservoir, which was installed on some late 1978 models



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Fig. Fig. 3 Cross-section of the brake master cylinder



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Fig. Fig. 4 You can remove excess brake fluid using an old turkey baster and a coffee can



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Fig. Fig. 5 Loosen the brake lines with a flare nut wrench ...



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Fig. Fig. 6 ... then disconnect the lines from the master cylinder



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Fig. Fig. 7 Loosen and remove the brake master cylinder retaining nuts ...



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Fig. Fig. 8 ... then pull the master cylinder free from the mounting studs

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

  1. Disconnect the negative battery cable. Disconnect and plug the hydraulic lines from the master cylinder.
  2.  
  3. Remove the retaining nuts and the lockwashers holding the cylinder to the cowl or the brake booster.
  4.  

If equipped with non-power brakes, disconnect the pushrod at the brake pedal.

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

To install:

  1. Position the master cylinder and torque the master cylinder mounting nuts to 22 ft. lbs. and the hydraulic lines to 18 ft. lbs. Refill (bench bleed master cylinder before installation-refer to the necessary procedure) the master cylinder, bleed the brake system and check the brake pedal free-play.
  2.  

On non-powered brakes, position the master cylinder on the cowl, making sure that the pushrod goes through the rubber boot into the piston. Reconnect the pushrod clevis to the brake pedal. If equipped with power brakes, install the master cylinder on the power booster.

BENCH BLEEDING



  1. Place the master cylinder in a vise.
  2.  
  3. Connect two lines to the fluid outlet orifices, and into the reservoir.
  4.  
  5. Fill the reservoir with brake fluid.
  6.  
  7. Using a wooden dowel, depress the pushrod slowly, allowing the pistons to return. Do this several times until the air bubbles are all expelled-do not let master cylinder assembly run dry.
  8.  
  9. Remove the bleeding tubes from the master cylinder, plug the outlets and install the caps.
  10.  

OVERHAUL





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Fig. Fig. 9 Exploded view of the brake master cylinder-early vehicles with bleeder screws



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Fig. Fig. 10 Exploded view of the integral reservoir master cylinder without bleeder screws



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Fig. Fig. 11 Exploded view of the removable reservoir brake master cylinder

Use this service procedure and exploded view illustrations as a guide for overhaul of the master cylinder assembly. If in doubt about overhaul condition or service procedure REPLACE the complete assembly with a new master cylinder assembly.

  1. Remove the master cylinder from the vehicle.
  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 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. 6-32 x 5 / 8 " 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 11 / 64 " drill and then thread them with a 1 / 4 "-20 tap. Position a thick washer over the hole to serve as a spacer and then thread a 1 / 4 "-20 X 3 / 4 " hex-head bolt into the insert and tighten the bolt until the insert is free.
  16.  
  17. Use only denatured alcohol or brake fluid and compressed air to clean the parts. slight rust may be removed with crocus cloth.
  18.  

Do not polish the aluminum bore of type A: cylinders with any type of abrasive. Never use any mineral based solvents (gasoline, kerosene, etc.) for cleaning. It will quickly deteriorate rubber parts.

  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 of the overhaul kit.
  10.  
  11. Coat the cylinder bore and the secondary piston's inner and outer seals with brake fluid. Assembly 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 hole in the bottom of the front reservoir. Torque 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 vehicle. Do so in the following manner:
  24.  
    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 (try the eraser end of a pencil), 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 the front end is tilted slightly upward. Repeat Steps d and e.
    12.  
    13. Place the diaphragm cover on the reservoir.
    14.  


Master cylinder overhaul on vehicles with power brakes is the same as above.

 
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