1998 GMC Envoy Owner's Manual - Page 274

Brakes Brake Fluid 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 off' your brake fluid. Adding brake fluid won't correct a leak. If you add fluid when your linings are worn, then 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 done on the brake hydraulic system. Your brake master cylinder reservoir is here. It is filled with DOT-3 brake fluid. There are only two reasons why the brake fluid level in the reservoir might go down. The first is that the brake fluid goes down to an acceptable level during normal brake lining wear. When new linings are put in, the fluid level goes back up. The other reason is that fluid is leaking out of the brake system. If it is, you should If you have too much brake fluid, it can spill if' on the engine. The fluid will burnthe engine is hot enough. You or others couldbe burned, and your vehicle could be damaged. Add brake fluid only when work is done on the brake hydraulic system. Refer to the Maintenance Schedule to determine when to check your brake fluid. See "Periodic Maintenance Inspections" in the Index. 6-27