Drum brakes use a simple mechanical process to stop your vehicle. Understanding how drum brakes work helps you diagnose problems and maintain them properly.
The Braking Process
When you press the brake pedal, the master cylinder converts pedal force into hydraulic pressure. Brake fluid travels through the brake lines to the wheel cylinder at each wheel. The pressure pushes pistons inside the wheel cylinder outward. These pistons force the brake shoes against the inner surface of the spinning drum. The friction between the shoes and the drum slows the wheel. When you release the pedal, return springs pull the shoes back to their resting position.
The Self-Servo Effect
Drum brakes have a feature called the self-servo effect. As the drum rotates, it pulls the brake shoes in the direction of rotation. This creates additional braking force without extra pressure on the pedal. The leading shoe gets pulled into the drum harder than the trailing shoe. This is why the leading shoe wears faster. The self-servo effect is one reason drum brakes are still used on rear axles.
Hydraulic System
The hydraulic system is what makes modern drum brakes work. The master cylinder generates pressure when you press the pedal. Brake fluid carries this pressure to the wheel cylinders. The system must be free of air and leaks. Air in the brake lines makes the pedal feel spongy. A fluid leak reduces braking pressure.
Parking Brake Integration
Drum brakes make it easy to integrate a parking brake. A cable pulls a lever inside the drum that forces the shoes against the drum mechanically. This holds the car in place without hydraulic pressure. The self-servo effect also helps the parking brake hold better on slopes.
Common Drum Brake Types
The most common type is the leading-trailing shoe design. One shoe acts as the leading shoe in forward motion, the other as the trailing shoe. Twin leading shoe and duo-servo designs are used in heavier vehicles for more braking force.
