At a quick glance most rear drum brakes appear very similar. Actually there are two major types in common use and dozens of variations. While they appear similar, they operate in different ways. Understanding the differences can help solve many mysterious brake problem.
Drum brake types
Leading/trailing brakes are most commonly used on smaller cars. The design gives balanced braking in light applications. Duo-servo brakes are used in heavier applications and where more aggressive braking is needed. Self adjustment and parking brake mechanisms have been removed from above photos for clarity.
Leading/trailing drum brake design
With the leading/trailing design, the wheel cylinder is usually at the very top. More significantly, there is a wedge style block where the shoes meet at the bottom. When the brakes are applied, fluid runs into the wheel cylinder and the shoes are pushed out against the brake drum. With leading/trailing brakes, both shoes move out with the same pressure. Rotation of the drum pushes down on the shoes. Since the bottom anchor is wedge shaped, the shoes spread outward and are pushed into the brake drums. The system depends on proper lubrication between the brake shoes, backing plate and lower anchor.
When the shoes on a leading/trailing system do not slide down properly, they tend to wear at the top. Shoes that slide down, but do not return when released, will wear at the bottom. Partial application can be due to lack of lubrication, worn or bent backing plates. Brakes in this condition do not perform very well. Rear brakes that do not work properly will cause the front brakes to do more stopping than necessary. This can result in rapid front brake pad wear, noise and warped front brake rotors.
Duo-servo brake system
With duo-servo brakes the wheel cylinder is mounted below a top anchor pin. The top of the brake shoes pivot on this pin and the bottom of the shoes are free to move, from side to side. When the wheel cylinder expands, the primary shoe is pushed forward and the secondary moves backward. Rotation of the brake drum causes the shoes to rotate at the bottom. This slight rotation forces the secondary shoe into full contact with the drum.
This rotational assist greatly increases the stopping force of duo-servo brakes. The harder the primary shoe applies the more the secondary is forced into the drum. This system relies on the shoes being able to move on the backing plate. This also requires proper lubrication between the shoes and backing plate.
Finding the problem
Six pads on the backing plate provide a lubricated slide surface for the shoes. These small pads are where high temperature brake grease must be applied, with either style brakes. Through years of use, or lack of lubrication, backing plates can wear. Wear creates grooves and cause the brake shoes to bind and apply unevenly.
Worn backing plates can also cause rear brakes to make noise, grab and even lockup. This will diminish braking on the rear and often show up as warped brake drums or front brake problems. Worn backing plates often have to be replaced. Sometimes specialty shops have the ability to weld and repair backing plates that are expensive, hard to replace or unavailable.
Drum brake backing plates can also be bent. Improper service, like prying brake drums off, often bends the backing plate. When the plate is bent, the shoes are not supported and do not slide properly. Noise, lockup, grabbing brakes and front brake issues are often the symptoms.
Bent backing plates can be identified with a dial indicator or run-out gauge. The gauge is attached to the axle with the tip on the edge of the backing plate. When the axle is rotated backing plate run-out can be measured. Bent brake backing plates can be replaced or specialty brake shops may be able to straighten them. Even new backing plates sometimes have slight run-out. Specialty brake shops can correct this run-out and often give enhanced braking performance.
With the above procedures many persistent problems associated with front and rear brakes are eliminated. Such attention to detail is a small part of what separates quality brake service from a brake job."