The Brake Servo works off the vacuum created by engine operation, or is powered by vacuum pumps. Sometimes known as power brakes or brake boosters, brake servos provide extra pressure when braking. This means that when the driver presses on the pedal, as well as the pressure from this action itself, the brakes are given a helping hand by the extra force created by the servo.
Correct functioning of the brake servo for any car is checked during the MOT, and if found to be significantly damaged or worn it must be replaced. It is advisable to only use high quality brake servos.