Brake Booster
A device that multiplies the force applied to the brake pedal, making it easier to apply the brakes using engine vacuum or a hydraulic pump.
The brake booster is a safety device located between the brake pedal and the master cylinder. Its primary function is to multiply the mechanical force applied by the driver's foot on the brake pedal, making it much easier to compress the hydraulic fluid and stop the vehicle. Without a brake booster, stopping a modern vehicle would require immense leg strength.
Most modern passenger vehicles use a vacuum-assisted brake booster. It consists of a metal canister divided into two chambers by a flexible rubber diaphragm. One chamber is connected to the engine's intake manifold via a vacuum hose, creating a constant vacuum. When you press the brake pedal, a valve allows atmospheric air into the other chamber, pushing the diaphragm forward and assisting the master cylinder piston.
If a brake booster fails—often due to a ruptured internal diaphragm or a leaking vacuum line—the booster will lose its power assist. The most common symptom is a very stiff or hard brake pedal that requires excessive force to slow the vehicle. Other signs include a hissing sound when pressing the brakes, engine hesitation or rough idling when the pedal is pressed (due to a vacuum leak), and increased stopping distances.
A vehicle with a failing brake booster is unsafe to drive. Replacing a damaged booster restores light, responsive pedal pressure and reliable stopping capability.