What is a Brake Servo?
The vacuum-powered helper that makes your brake pedal light and easy to press.
Simplified animation — not to scale.
In plain English
A brake servo — also called a brake booster — is a round, drum-sized canister mounted between the brake pedal and the master cylinder. It uses engine vacuum (or an electric pump on some hybrids and EVs) to multiply the force your foot applies, so you can stop a heavy car with a gentle press of the pedal.
"Imagine trying to squirt ketchup out of a glass bottle by hand versus using a squeezy plastic one — the servo is the soft bottle that does most of the work for you."
How it works
When you press the brake pedal, a valve inside the servo opens and lets atmospheric pressure push against a diaphragm on one side while engine vacuum pulls on the other. The resulting force is passed on to the master cylinder, giving you power-assisted braking. When you release the pedal, the valve closes and the system resets, ready for the next stop.
Signs of trouble
- ⚠Very hard brake pedal that requires lots of leg strength
- ⚠Longer stopping distances than usual
- ⚠Hissing noise from behind the dashboard when braking
- ⚠Brake pedal slowly sinks to the floor under pressure
- ⚠Engine-stall feeling when pressing the brake at idle
£160 – £450 fitted
Always get a written quote. Prices vary by car, region, and parts brand.