How your brakes work
Brake Pedal The brake pedal is directly attached to the master cylinder. Pedal pulsation, excessive pedal travel, a “soft” or “hard” pedal can be indicators of serious problems, including a leak in the hydraulic system, low fluid levels, or unevenly worn shoes or pads.

Master Cylinder The master cylinder acts as a holding tank for brake fluid until it is needed. When the brake pedal is depressed, the master cylinder forces fluid to each of the vehicle's wheels. Wear on the master cylinder's moving parts may allow brake fluid to leak, causing unreliable stopping or possible system failure.

Combination Valve A vehicle's wheel can lock up if the front and rear brake systems are not working together properly. Comprised of a metering valve, proportioning valve, and brake warning light, the combination valve helps regulate the amount of pressure on each set of wheels – making sure both front and rear brakes are applied at the same time.

Wheel Cylinder The wheel cylinder is a critical element in the drum brake assembly. It contains fluid-activated pistons that push the shoes against the drums to slow the wheels. The wheel cylinder is also the source of many brake problems. If brake fluid leaks from the wheel cylinder, the vehicle could experience unreliable stopping, damage to new brake shoes, or partial brake system failure. A sticking wheel cylinder may cause brake drag, excessive pedal effort, and reduced braking efficiency.

Drum Brake Assembly A drum brake assembly is used to bring the rear wheels of most vehicles to a stop. Fluid pressure from the master cylinder causes the wheel cylinder to push the brake shoes against the brake drums which are attached to the vehicle's rear wheels. The friction between the stationary shoes and the revolving drums causes the drums to slow and stop the rear wheels. Worn drums and shoes, however, can cause unreliable stopping, excessive pedal effort, or brake pedal pulsation.

Disc Brake Assembly Because a disc brake assembly can absorb more heat than a drum brake assembly, most cars use disc brakes for their front brake systems. When the brake pedal is pushed, brake fluid from the master cylinder compresses the brake pads against the rotors attached to the vehicle's front wheels. The friction between the stationary pads and the revolving rotors causes the rotors and wheel to slow and stop.
In day-to-day driving, these rotors and pads are subject to much abuse, and should be checked periodically for wear. Faulty disc brakes can cause excessive pedal travel, pumping or fighting pedal, vibration during braking action, and brake failure. Artwork supplied courtesy of Wagner Division, Cooper Industries.