1990 GMC Sierra brake light on dash on

Due to the wide range of weight distribution you might find due to loading, most minivans and pickup trucks have height-sensing proportioning valves connected to their rear axles. Proportioning valves limit brake fluid flow to the rear wheels to reduce rear-wheel lockup under hard braking. If your truck is jacked up with the rear axle hanging down, that mimics no load in the rear with hard braking. You may need to install the wheels and set the truck on the ground, or you can disconnect the linkage from the valve assembly to the axle housing to let more fluid flow.