The brake servo unit, if fitted, is an aid to the hydraulic braking system. With the operation of the servo unit less force is required on the brake pedal.
The servo unit is situated in the braking system and has a connecting rod that runs between the brake pedal, the master cylinder and fluid reservoir and is commonly found in the engine compartment.
The brake servo unit is a bulbous cylinder which contains a central diaphragm. On each side of the diaphragm there are two separate chambers The cylinder has a vacuum feed from the engine manifold this maintains a vacuum in both chambers. When the brake pedal is pressed an influx of external air is sucked into one side of the diaphragm, the air pressure rises as the vacuum is broken which pushes the diaphragm to assist the movement of the master cylinder which forces the hydraulic fluid through the piping to the slave cylinders on the brakes of each wheel. When the footbrake is released the vacuum is restored to the chamber.
Signs that there's a problem with the brake servo unit can be that the pedal becomes heavy and requires increased force to operate the brakes.