Cars are often equipped with drum brakes in the rear and disc brakes in the front for cost reasons. Drum brakes are cheaper for the manufacturer to install.
Drum brakes work on the same principle as disc brakes. Shoes push against a moving surface and the friction lowers the rotational speed of the wheel. The basic construction of a drum brake is a cylinder (the drum) which contains two brake shoes and a piston. The drum rotates on the same axis as the wheel. The shoes face the inside of the cylinder and the piston sits between them. There are also a number of springs in place to pull the shoes back away from the drum when the brakes aren't active and some levers for the emergency brake.
When the brake pedal is pushed, brake fluid from the master cylinder enters the piston and forces the shoes apart, pushing them against the drum. The emergency brake system is much easier to implement in cars with drum brakes. The power for the braking comes from a cable which, when pulled tight, forces the brake shoes apart. This is important, since emergency brakes must operate even if the hydraulic system isn't functional.
The brakes are located on the inside of the wheels (where the tire is attached), and are attached to the hub. The brakes have many parts inside, the most obvious of which are the shoes. They are semicircular bits of metal with pads on the outside. At one end, they will be connected by the piston, and at the other by the adjuster mechanism.
Drum brakes lose effectiveness in sustained braking because they quickly overheat. Disc brakes run much cooler comparatively, and will be much more effective at slowing you while going down hills. The danger of overheating means that drum brakes must have less braking power than a disc brake. Disc brakes can put a lot more hydraulic force into stopping the car without damaging the brakes or fading.
Because friction is involved in this braking system, there will be wear. It is important to keep the gap between the drum and the shoes constant, so that there is no noticeable change in the action of the brake pedal. Modern drum brakes have a self-adjusting mechanism that automatically moves the shoes closer to the drum as they wear. As shoes wear out, a small metal bit touches the drum, making a squeaking noise while braking lightly. This is an indication that the shoes will need to be changed soon.
If the brake shoes are replaced at the proper time (before they are completely worn away), they should be the only thing that will need servicing. Generally, it is a good idea to replace the shoes when there is between 1/32" and 1/16" of material left. If the shoes are allowed to remain in poor shape for too long, rivets in the shoe may gouge the inside of the drum, producing unpleasant noises, and less effective brakes. Contamination due to oil or other fluids or erosion due to burning are also factors in brake wear. Sudden, heavy braking greatly increases wear and can burn the surface of the shoes or the inside of the drum.
Should you have to replace your brake shoes, it is an inexpensive repair. Cost should be in the range of $30 each (Canadian) plus installation.
Sources:
- http://www.howstuffworks.com/drum-brake.htm
- http://www.raybestos.com/OnLineTraining/cpt-014.htm
- http://www.edmunds.com/ownership/techcenter/articles/43857/article.html