A brake servo, also known as a brake or vacuum booster, works by creating a partial vacuum within, which then increases the force applied to the main cylinder. With a brake servo, the brake pedal first presses an attached rod, which then allows air to enter the booster while closing the vacuum. Pressure then increases on the rod that connects to a rod inside the master cylinder.
The brake servo has become more common in cars as disc brakes have replaced drum brakes as the standard setup in vehicles. Disc brakes make it necessary for cars to have power brakes to remove a majority of the force that a driver needs to exert to stop the car.
Inside the brake servo system, a vacuum multiplies the force that is exerted by the driver on the brake pedal. The outward appearance of a brake servo is a canister that contains a diaphragm, a valve and is usually constructed from metal. Attached to the brake servo is also a check valve, which limits the direction of air to outward only in order to eliminate the risk of losing brake function while the car is operating.
The brake servo system was invented in 1927 by Albert Dewandre in Liege, Belgium.