GM Full-Size Trucks 1980-1987 Repair Guide

Master Cylinder

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BRAKE FLUID LEVEL CHECK



See Figures 1, 2 and 3

Chevrolet and GMC trucks are equipped with a dual braking system, that allows the truck 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.
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  3. Be sure that the vehicle is resting on a level surface.
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Fig. Fig. 1: If the master cylinder reservoir is not marked, make sure the fluid is within 1/4 in. reservoir wall top

  1. Carefully pry the clip from the top of the master cylinder to release the cover. Some late models have a plastic reservoir which do not have a clip. The plastic type caps can be removed by grasping the cap on both sides while pulling up on one side. Keep a rag handy just in case some fluid spills. Brake fluid will remove the paint finish so be careful.
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Fig. Fig. 2: Some master cylinders are equipped with a plastic reservoir. Fluid must be kept above the min. lines, but 1/4 in. below the top

  1. The fluid level should be approximately 1 /4 in. from the top of the master cylinder or, in the case of the plastic reservoir, above the minimum marks on reservoir sides. The fill marks on the plastic cylinders generally resemble a pair of steps embossed in the plastic. Be sure to use a high quality brake fluid such as General Motors Supreme No. 11 Hydraulic Brake Fluid, or fluids meeting DOT 3 specifications should be used.
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Fig. Fig. 3: Plastic reservoir lids are snapped into position and may be carefully pried off by hand

Do not use brake fluid that has been used before or has been sitting in a container that has been open for a long period. Brake fluid will absorb moisture in an open or poorly sealed container. Contaminated brake fluid will greatly shorten the life of brake components. It is normal for the fluid level to fall slightly as the disc brake pads wear. 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.
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  3. Push the clip back into place and be sure that it seats in the groove on the top of the cover.
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WARNING
Brake fluid damages paint. It also absorbs moisture from the air; never leave a container or the master cylinder uncovered any longer than necessary. All parts in contact with the brake fluid (i.e. master cylinder, and its lid, hoses, plunger assemblies, etc.) must be kept clean, since any contamination of the brake fluid will adversely affect braking performance.

 
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