GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Brake Master Cylinder


See Figure 1

Chevrolet and GMC trucks are equipped with a dual braking system, allowing a 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 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 portion is unaffected.

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Fig. Fig. 1: Most master cylinder reservoirs are marked with MIN level fill lines


Use only heavy-duty Delco Supreme 11 or DOT 3 brake fluid.


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 (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 2

It should be obvious how important the brake system is to safe operation of your vehicle. The brake fluid is key to the proper operation of the brake system. Low levels of fluid indicate a need for service (there may be a leak in the system or the brake pads may just be worn and in need of replacement). In any case, the brake fluid level should be inspected at least during every oil change, but more often is desirable. Every time you open the hood is a good time to glance at the master cylinder reservoir.

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Fig. Fig. 2: When adding brake fluid to the master cylinder use only clean, fresh fluid from a sealed container

To check the fluid level on most vehicles covered by this manual, you may peer through the side wall of the reservoir and observe the level in relation to the markings. If the reservoir is opaque, simply unsnap and lift off the reservoir cover, to check the fluid level; it should be within 1 / 4 of the tops of the reservoir walls. When making additions of brake fluid, use only fresh, uncontaminated brake fluid which meets or exceeds DOT 3 standards. Be careful not to spill any brake fluid on painted surfaces, as it will quickly eat the paint. Do not allow the brake fluid container or the master cylinder reservoir to remain open any longer than necessary; brake fluid absorbs moisture from the air, reducing its effectiveness and causing corrosion in the lines.