Under normal conditions–and when adjusted properly–rear brake shoes will last two to three times longer than the front pads. This is because the proportioning valve applies 60 percent of the braking force to the front wheels. Rear brake shoes are made of an asbestos material that is glued and riveted to a metal backing plate.
Extending the Life
To promote the rear brake shoes’ life, the brake drum should be removed every time the front pads are replaced. The brake drum and rear shoes and hardware should be cleaned to prevent abrasive material wearing on the shoes. The rear brake shoes should be adjusted so that they just barely rub the drum when the drum is turned. The emergency brake should be checked and adjusted to make sure that the it releases all the way and does not stick, causing the rear shoes to rub. All the hardware should be checked for broken springs or pieces. Another thing that can cause rapid rear break wear are leaking rear wheel cylinders. Check the rear wheel cylinders for leaks by looking at the cylinder rubber boots. Peel them back a small amount to see if any fluid runs out. If there is any sign of leakage, replace the wheel cylinders.
Another mistake that accelerates wear on the rear brake shoes is when they are installed in the wrong position. Looking at the brake shoes, notice the location of the braking material on the metal backing pad. The front shoe is different than the rear shoe. It is imperative that the low shoe with the most bare metal backing on top is located in the front toward the engine. The high shoe is located in the rear. They will fit either way, although the wear rate will be dramatically different.
Inspecting for Wear
When inspecting brake shoes for wear, the rivets that hold the brake material on to the metal backing plate are the indicator. When the braking material becomes worn to the point that it is close to the rivets, it is time to replace them. Inspect the brake lining for deep grooves that could cause less braking surface touching the drum. If this is indicated, the drum will also be grooved and must be machined to eliminate this condition. The brake shoes should also be checked for cracks and missing pieces. Check the brake shoes for contamination from leaky wheel cylinders.
Inspecting the Drums
Worn or warped brake drums will cause rapid brake wear and possibly a pulsation in the brake pedal when the brakes are applied. An easy way to check for warpage is to turn the rear axle with the drum on the brakes and see if it hangs up or can be heard rubbing in one location. The thickness of the drums should be checked prior to turning. If the drums are too thin, they should be replaced.
Adjustable Proportioning Valves
Adjustable proportioning valves are available for performance upgrades in the braking system. These can be adjusted to apply as much or as little braking force on the rear brakes as necessary. If they are adjusted to apply too much braking force, the shoes will wear out faster.