Drum Brakes

Hydraulic wheel cylinders push curved brake shoes outward against the inside surface of a rotating drum. While disc brakes are now standard on the front axle of virtually every vehicle, drum brakes are still used on the rear axle of many trucks, economy vehicles, and fleet vehicles. They are cheaper to manufacture and effective for rear braking duty where less stopping force is required.

The leading shoe — the shoe whose friction surface contacts the drum in the direction of forward rotation — is self-energizing. As the drum rotates, it tries to drag the leading shoe along with it, wedging it harder into contact. Think of it like dragging your hand along a spinning merry-go-round — the rotation pulls your hand along and pushes it harder into the surface. This self-energizing effect multiplies braking force beyond what hydraulic pressure alone provides. The trailing shoe does not get this benefit and wears more slowly as a result.

Drum brakes use automatic self-adjusters that compensate for lining wear by moving the shoes incrementally closer to the drum as the linings wear down. Most designs activate during reverse braking stops — every time you back up and hit the brakes. A seized or inoperative self-adjuster causes the pedal to gradually require more travel as the linings wear because the shoes never adjust outward to compensate. The parking brake also relies on proper shoe adjustment. Regular inspection and lubrication of self-adjuster components during brake service prevents these issues.

Pull the drum and inspect the shoe lining thickness — replace shoes when lining thickness reaches the minimum specification or the wear indicators. Check the wheel cylinder for leaks by pulling back the rubber boots on each end — any fluid seepage means the cylinder seals are failing and the cylinder must be replaced or rebuilt. Inspect all springs and hardware for stretching, corrosion, and damage. Worn springs cause improper shoe return, dragging, and noise. Check the drum inner surface for scoring, heat cracks, and out-of-round conditions. Measure the drum inner diameter — if it exceeds the maximum diameter specification stamped on the drum, replace it.