The GMC Sierra pickup truck utilizes front disc brakes and rear drum brakes for stopping. Both the front and rear brakes have a self-adjusting mechanism, but if the rear drum brakes fall too far out of adjustment, the mechanism cannot function properly. Fortunately this is an easy problem to repair. During routine maintenance, you should check the rear brake adjustment by raising the vehicle and turning the rear wheels by hand. With the brake released, the wheel should turn freely. With the pedal lightly depressed, you you feel drag on the wheel. If these conditions aren't present, you should adjust the brakes.
Raise the rear of the vehicle with an automotive jack, and support both sides using jack stands.
Locate the adjusting hole on the rear of the brake drum backing plate. This is the piece that covers the drum on the inside part of the wheel. The inspection hole should have a rubber grommet installed. Use a flat blade screwdriver to pry out the grommet.
Insert the flat blade screwdriver into the inspection hole to make contact with the the star wheel on the brake shoe adjuster. The adjuster expands or retracts as you rotate the wheel, placing more or less force on the brake shoes. To expand the brake shoes, place the screwdriver against the top of the star wheel and push it forward to rotate. To retract the brake shoes, place the screwdriver against the bottom of the star wheel and push it forward to rotate.
Expand the brake shoes until it becomes difficult to rotate the wheel by hand. The shoes are now rubbing against the drum.
Contract the brake shoes by rotating the star wheel 20 clicks. Remember, to contract the brake shoes, press the screwdriver against the bottom of the star wheel. Ensure that the wheel turns freely. Repeat the procedure on the opposite wheel. Replace both inspection hole covers.
Lower the vehicle from the jack stands. Locate an area where you can drive the truck, while making frequent, quick stops. Place the truck in reverse and begin to drive backwards. After a few feet, press the brake pedal firmly to quickly stop the vehicle. Drive forward and repeat the rapid stop. Repeat the back and forth braking several times. This is what activates the self-adjusting mechanism on the brake shoes.
