Chevrolet Full Size Cars 1979-1989

Master Cylinder

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Vehicles covered by this guide are equipped with front disc brakes. Master cylinders on vehicles equipped with disc brakes do not have a check valve in the cylinder front outlet port. If one is installed, the front discs will quickly wear out due to residual hydraulic pressure holding the pads against the rotor.

REMOVAL & INSTALLATION





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Fig. Fig. 1 Common master cylinder mounting non-power brake vehicles



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Fig. Fig. 2 On vehicles equipped with power brakes, the master cylinder assembly is mounted to the booster instead of the cowl



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Fig. Fig. 3 It is best to use a line wrench to loosen the hydraulic line fittings



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Fig. Fig. 4 Loosen and remove the master cylinder retaining nuts

  1. Use a siphon or turkey baster to remove the brake fluid from the master cylinder reservoir.
  2.  
  3. Loosen the fittings and disconnect the hydraulic lines from the master cylinder assembly. Immediately plug or cap all openings to prevent system contamination or excessive fluid loss.
  4.  
  5. Remove the retaining nuts and the lockwashers holding the cylinder to the cowl or the brake booster, as applicable.
  6.  

If equipped with non-power brakes, remove the clevis pin and disconnect the pushrod at the brake pedal.

  1. Remove the master cylinder, gasket and, if applicable, the rubber boot. To install:
  2.  
  3. Position the master cylinder making sure the gasket and rubber boot are in position.
  4.  

On non-powered brakes, position the master cylinder on the cowl, making sure that the pushrod goes through the rubber boot into the piston.

  1. Secure the master cylinder using the retaining nuts, then tighten the nuts to 22 ft. lbs. (30 Nm).
  2.  
  3. If equipped with non-power brakes, reconnect the pushrod at the brake pedal and secure using the clevis pin.
  4.  
  5. Remove the plugs or caps, then reconnect the hydraulic lines and tighten the fittings to 18 ft. lbs. (24 Nm).
  6.  
  7. Refill the master cylinder with fresh brake fluid, then properly bleed the hydraulic brake system and check the brake pedal free-play. Make sure there is a firm brake pedal before attempting to move the vehicle.
  8.  

MASTER CYLINDER BENCH BLEEDING



  1. Install plugs in the outlet ports.
  2.  
  3. Carefully 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 bubbles disappear.
  10.  
  11. Loosen the vise and position the cylinder so the front end is tilted slightly upward. Repeat Steps 4 and 5, as necessary.
  12.  
  13. Place the diaphragm cover on the reservoir.
  14.  

OVERHAUL





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Fig. Fig. 5 Exploded view of a dual circuit master cylinder assembly note that most 1980 and later vehicles are not equipped with retaining balls on the reservoir cap



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Fig. Fig. 6 Moraine cast iron master cylinder with integral reservoir used on 350 diesel engine vehicles



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Fig. Fig. 7 Cross-sectional view of a master cylinder assembly



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Fig. Fig. 8 Exploded view of a late-model master cylinder assembly equipped with a removable reservoir

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. In some cases, a rebuilt master cylinder assembly may cost little more or the same as a rebuild kit.

  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 remaining 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, but be careful not to score or damage the bore.
  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 × 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 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 × 3 / 4 in. 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.  


WARNING
Do not polish a master cylinder aluminum bore 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. 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 or small wooden dowel, 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 inch lbs. (3.0-4.5 Nm).
  20.  
  21. Install the reservoir diaphragm and cover.
  22.  
  23. It will save time later if the master cylinder assembly is bench-bled before installation in the vehicle. Please refer to the bench bleeding procedure found earlier in this section.
  24.  

 
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