A pair of GMC Drum Brake Shoes is on each drum brake assembly on this brand of trucks. There is a smaller forward one and a slightly larger one facing the rear of the drum brake in each assembly to stop the GMC Truck.
The drum brake shoes are made from a composite material that is designed to turn to dust slowly when creating friction against the hard steel drum surface. This composite material on the drum brake shoe is mounted to a hard steel backing plate and held there by a super strong adhesive and rivets. It is the rivets that will make first contact with the drums when the composite material has worn away and it is time for a routine brake job on your GMC truck. This will create a metal to metal grinding sound that is indicating the drum's surface is being scared. This damage will have to be removed by a process called turning the drums.
The removal of the GMC drum brake shoes requires that numerous springs to be removed. It is these springs that not only hold the drum brake shoes in place but also retract them once the hydraulic pressure is released from the braking action. Each of these springs, usually six of them, have to be placed back on the drum brake assembly in the correct order for this braking system to function correctly. To accomplish this most people that perform this repair procedure take both drums off but only dissemble one of the brake assemblies at a time. This makes it possible for the drum brake shoes to be correctly mounted so they can function as they are designed too on your GMC truck.