The wheel cylinder supplies the muscle for a drum brake. Most wheel cylinders are relatively simple: a pair of horizontally opposed pistons mounted inside a cast iron or aluminum housing. A brake line or hose from the master cylinder routes hydraulic pressure to the wheel cylinder, which pushes the pistons out. This forces the brake shoes against the drum to apply the brake. When the brake pedal is released, return springs on the shoes retract them from the drum and push the pistons back into the wheel cylinder.

Each piston has a cup-shaped seal that faces inward. When hydraulic pressure is exerted against the piston and seal, the lip of the seal is pushed outward and seals tightly against the inside bore of the wheel cylinder. A dust boot that keeps out brake dust and other contaminants is also on the outside of each piston.

Over time, normal wear, roughness or pitting in the bore, or nicks or cracks in the piston seal, can create paths for pressure to be lost. And once a wheel cylinder starts to leak brake fluid, it's bad news for the brakes. That's why the wheel cylinders should always be inspected when the brakes are relined - even if it's only the front disc brakes that are receiving new pads.

Brake fluid leaking from a wheel cylinder will foul the brake shoes, causing a loss of friction. This, in turn, can make the brakes pull (toward the side with the good wheel cylinder), causing uneven braking and/o...