All vehicles employ a leading/trailing type of drum brake, in which there are two curved brake shoes supported by an anchor plate and wheel cylinder. When the brake pedal is depressed and hydraulic pressure is delivered to the wheel cylinder, the wheel cylinder expands to force the shoes against the drum.
Friction between the brake shoes and the drum causes the vehicle to slow and stop. When the brake pedal is released, the brake shoe return springs move the brakes away from the drum. If the lining on the brakes becomes contaminated or the lining/drum becomes grooved, the engagement of the brakes and drum will become very harsh, causing the brakes to lock up and/or squeal. If the brake shoes on one wheel contact the drum before the same action occurs in the other wheels, the brakes will pull to one side when applied.