It is the GMC drum brake wheel cylinders that move the brake shoes against the drums so friction can be made. These are the braking components that use the hydraulic pressure being generated by the driver pushing on the brake pedal so the vehicle can be brought to a safe and controlled stop. If you have drum brakes, there are GMC drum brake wheel cylinders on each side in the brake assemblies. The part that fails on the cylinder is the seals used to hold the hydraulic fluid in place in the pistons of the cylinder itself. There are two such seals per drum brake wheel cylinder on the GMC trucks. If a leak occurs, the leaking brake fluid will form a puddle on the pavement near the inside of the tire. The pistons of the drum brake wheel cylinder push out on the upper portions of the brake shoes and why this brake component is located on the backing plate in the center at the top of it. The drum brake wheel cylinder can only be replaced once the brake shoes are removed from the backing plate. Before the cylinder holds down bolts are loosened, the brake line or hose must be taken off. This will prevent any damage from occurring to it. Once the removal of the old part is completed on your GMC truck, the new drum brake wheel cylinder can be put into place. With the brake shoes mounted and the drum put back in place the brake line must then be bled of any air that entered it when it was removed. This is accomplished with pressure on the system and the nipple on top of the cylinder being opened. This will allow your GMC truck to have full braking power again.