GM Full-Size Trucks 1970-1979 Repair Guide

Master Cylinder

Print

REMOVAL & INSTALLATION



See Figures 1, 2, 3, 4 and 5



Click image to see an enlarged view

Fig. Fig. 1: Use a turkey baster to remove the brake fluid from the reservoir



Click image to see an enlarged view

Fig. Fig. 2: Disconnect and plug the fluid lines from the master cylinder



Click image to see an enlarged view

Fig. Fig. 3: Unfasten the master cylinder mounting nuts ...



Click image to see an enlarged view

Fig. Fig. 4: ... then remove the master cylinder from the power brake booster



Click image to see an enlarged view

Fig. Fig. 5: Manual and power brake master cylinder mountings

  1. Using a clean turkey baster or equivalent, remove as much of the brake fluid as possible from the reservoir and place in a suitable container.
  2.  
  3. Using a clean cloth, wipe the master cylinder and its lines to remove excess dirt and then place cloths under the unit to absorb spilled fluid.
  4.  


WARNING
Clean master cylinder parts in alcohol or brake fluid. Never use mineral based cleaning solvents such as gasoline, kerosene, carbon tetrachloride, acetone, or paint thinner as these will destroy rubber parts.

  1. Remove the hydraulic lines from the master cylinder and plug the outlets to prevent the entrance of foreign material.
  2.  
  3. Disconnect the brake pushrod from the brake pedal (non-power brakes).
  4.  
  5. Remove the attaching bolts and remove the master cylinder from the firewall or the brake booster. Remove the master cylinder from the booster.
  6.  
  7. Connect the pushrod to the brake pedal with the pin and retainer.
  8.  
  9. Connect the brake lines and fill the master cylinder reservoirs to the proper levels.
  10.  
  11. Bleed the brake system.
  12.  
  13. If necessary, adjust the brake pedal free-play.
  14.  

OVERHAUL



See Figure 6



Click image to see an enlarged view

Fig. Fig. 6: Exploded view of the master cylinder internal components

In most years, there are two sources for master cylinders: Delco-Moraine and Bendix. The Bendix unit can readily be identified by the secondary stop bolt on the bottom, which is not present on the Delco-Moraine unit. Master cylinders bearing identifying code letters should only be replaced with cylinders bearing the same code letters. Secondary pistons are also coded by rings or grooves on the shank or center section of the piston, and should only be replaced with pistons having the same code. The primary pistons also are of two types. One has a deep socket for the pushrod and the other has a very shallow socket. Be sure to replace pistons with identical parts. Failure to do this could result in a malfunction of the master cylinder.

  1. Remove the secondary piston stop screw (if equipped) which is located at the bottom of the master cylinder front reservoir.
  2.  
  3. Position the master cylinder is a vise covering the jaws with cloth to prevent damage.
  4.  
  5. Remove the lockring from the inside of the piston bore. Once this is done, the primary piston assembly may be removed.
  6.  
  7. The secondary piston, piston spring, and the retainer may be removed by blowing compressed air through the stop screw hole. If compressed air is not available, the piston may be removed with a small piece of wire. Bend the wire 1 / 4 in. from the end into a right angle. Hook this end to the edge of the secondary piston and pull it from the bore. The brass insert should not be removed unless it is being replaced.
  8.  
  9. Inspect the piston bore for corrosion or other obstructions. Make certain that the outer ports are clean and the fluid reservoirs are free of foreign matter. Check the by-pass and the compensation ports to see if they are clogged.
  10.  
  11. Remove the primary seal, seal protector, and secondary seals from the secondary piston.
  12.  

Clean all parts in denatured alcohol or brake fluid. Use a soft brush to clean metal parts and compressed air to dry all parts. If corrosion is found inside the housing, either a crocus cloth or fine emery paper can be used to remove these deposits. Remember to wash all parts after this cleaning. Be sure to keep the parts clean until assembly. All rubber parts should be clean and free of fluid. Check each rubber part for cuts, nicks or other damage. If there is any doubt as to the condition of any rubber part, it is best to replace it.

Since there are differences between master cylinders, it is important that the assemblies are identified correctly. There is a two-letter metal stamp located at the end of the master cylinder. The stamp indicates the displacement capabilities of the particular master cylinder. If the master cylinder is replaced, it must be replaced with a cylinder with the same markings.

  1. Install the new secondary piston assembly.
  2.  

The seal which is nearest the flat end has its lips facing toward the flat end. On Delco units, the seal in the second groove has its lips facing toward the compensating holes of the secondary piston. On Bendix units, the seal is an O-ring.

  1. Install the new primary seal and seal protector over the end of the secondary piston opposite the secondary seals. It should be positioned so that the flat side of the seal seats against the flange of the piston with the compensation holes.
  2.  

The seal protector is not used on 1977-79 models.

  1. Install the complete primary piston assembly included in every repair kit.
  2.  
  3. Coat the master cylinder bore and the primary and secondary seals with brake fluid. Position the secondary seal spring retainer into the secondary piston spring.
  4.  
  5. Place the retainer and spring over the end of the secondary piston so that the retainer is placed inside the lips of the primary seal.
  6.  
  7. Seat the secondary piston. It may be necessary to manipulate the piston to get it to seat.
  8.  
  9. Position the master cylinder with the open end up and coat the primary and secondary seal on the primary piston with brake fluid. Push the primary piston into the bore of the master cylinder. Hold the piston and position the lockring.
  10.  
  11. Still holding the piston down, install and tighten the stop screw to a torque of 25-40 inch lbs.
  12.  
  13. Install the reservoir cover and also the cover on the master cylinder and its retaining clip.
  14.  
  15. Bleed the master cylinder of air. Do this by positioning the cylinder with the front slightly down, filling it with brake fluid, and working the primary piston until all the bubbles are gone.
  16.  

 
label.common.footer.alt.autozoneLogo