2008 GMC Yukon Owner's Manual - Page 410

It is not a good idea to top off the brake fluid. Adding brake fluid will not correct a leak. If fluid is added when the linings are worn, there will be too much fluid when new brake linings are installed. Add or remove brake fluid, as necessary, only when work is done on the brake hydraulic system. Checking Brake Fluid The brake fluid can be checked without taking off the cap by looking at the brake fluid reservoir. The fluid level should be above MIN. If it is not, have the brake hydraulic 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. When the brake fluid falls to a low level, the brake warning light will come on. See Brake System Warning Light on page 3-44. Refer to the Maintenance Schedule to determine when to check the brake fluid. See Scheduled Maintenance 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. 5-42