1994 GMC Sonoma Owner's Manual - Page 247

Brakes Brake Master Cy/incier Your brake master cylinder is here. is It filled with DOT-3 brake fluid. There areonly two reasons why the brake fluid levelin your master cylinder might go down. The firstis that the brake fluid goes down toan acceptable level duringnormal brake lining wear. When new linings are put in, the fluid level goes back The other reason is that fluid is leaking out the up. of brake system. If it is, you should have your brake system fixed, since a leak means that sooner or later your brakes won't work well, or won't work at all. So, it isn't a good idea to "top your brake fluid.Adding brake fluid off' won't correct a leak. you add fluid when your linings areworn, then If you'll have too much fluid when you get new brake linings. You should add (or remove) brake fluid,as necessary, only when work is doneon the brake hydraulic system. Refer to the Maintenance Schedule to determine to check your when brake fluid. See "Periodic Maintenance Inspections" the Index. in 6-29