2004 GMC Sierra 1500 Pickup Owner's Manual - Page 449

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. After work is done on the brake hydraulic system, make sure the level is above the MIN but not over the MAX mark. 5-49