2006 GMC Sierra 1500 Pickup Owner's Manual - Page 460

So, it is not a good idea to top off your brake fluid. Adding brake fluid will not correct a leak. If you add fluid when your linings are worn, then you will 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. Checking Brake Fluid You can check the brake fluid without taking off the cap. Just look at the brake fluid reservoir. The fluid level should be above MIN. If it is not, have your brake system checked to see if there is a leak. {CAUTION: If you have too much brake fluid, it can spill on the engine. The fluid will burn if the engine is hot enough. You or others could be burned, and your vehicle could be damaged. Add brake fluid only when work is done on the brake hydraulic system. See "Checking Brake Fluid" in this section. Refer to the Maintenance Schedule to determine when to check your brake fluid. See Scheduled Maintenance (Gasoline Engine) on page 6-4. After work is done on the brake hydraulic system, make sure the level is above the MIN but not over the MAX mark. 5-48