Periodically, the GMC disc brake pads on your truck will have to be replaced because they will be worn out. This is a natural action from the result of the friction caused by them being pressed against the rotors of the truck. The disc brake pads are held into place by being mounted in the calipers of the braking system on your GMC truck. There is an inner and an outer pad that work together to apply pressure to both sides of the rotor. The materials the disc brake pads are made out of are softer than the metal discs and will wear over time of the pressure being applied. This is done on purpose, since it is less expensive to replace the pads than the rotors on your truck. The GMC disc brake pads can easily be replaced without the hydraulic system of the braking system being opened. Once the caliper is removed, the old disc brake pads can be removed and the new one put into place. The backing plates of the pads can then be observed to be made out of metal. If the pads wear to the point where the backing plates come in contact with the rotors then damage to the rotors will occur since these are made out of metal. That situation should be avoided to keep cost low on a routine brake job on your GMC truck. Since the new disc brake pads are thicker than the ones being taken off, the piston in the caliper has to be pushed back into the cylinder for them to fit. To do this with the least amount of resistance, the cover on the master cylinder should be taken off. Make sure to clean up any overflow of brake fluid when the piston is being pushed back into the cylinder or the paint job on your GMC truck might come off.