GM Full-Size Trucks 1970-1979 Repair Guide

Master Cylinder



When adding fluid to the brake master cylinder, always use Delco Supreme fluid part no. 1052535 or equivalent DOT-3 brake fluid from a clean, sealed container.

Brake fluid damages paint. It also absorbs moisture from the air; never leave a container or the master cylinder uncovered longer than necessary. All parts in contact with the brake fluid (master cylinder, hoses, plunger assemblies and etc...) must be kept clean, since any contamination of the brake fluid will adversely affect braking performance.


See Figure 1

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Fig. Fig. 1: Remove the master cylinder lid, then check to see if the brake fluid level is correct

Chevrolet and GMC trucks are equipped with a dual braking system, allowing a vehicle to be brought to a safe stop in the event of failure in either the front or rear brakes. The dual master cylinder has two entirely separate reservoirs, one connected to the front brakes and the other connected to the rear brakes. In the event of failure in either portion, the remaining part is not affected.

  1. Clean all of the dirt from around the cover of the master cylinder.
  3. Be sure that the vehicle is resting on a level surface.
  5. Carefully pry the clip from the top of the master cylinder to release the cover.
  7. The fluid level should be approximately 1 / 8 in. from the top of the master cylinder. If not, add fluid until the level is correct. Only high quality brake fluids, such as General Motors Supreme No. 11 Hydraulic Brake Fluid, or or equivalent fluids meeting DOT 3 specifications should be used.

It is normal for the fluid level to fall slightly as the disc brake pads wear, on 1971 and later trucks. However, if the level drops significantly between fluid level checks, or if the level is chronically low, the system should be examined for leakage.

  1. Install the cover of the master cylinder. On most models there is a rubber gasket under the cover, which fits into two slots on the cover. Be sure that this is seated properly.
  3. Push the clip back into place and be sure that it seats in the groove on the top of the cover.