The brake booster module has an inlet (12) for the master brake cylinder connection and an outlet (13) to the wheel brake cylinders. A connection (17) to the hydraulic pump (5) and pressure reservoir (6) and a top up reservoir (4) connection (18) completes the module. The module contains control valves to connect the servo pressure to the wheel brakes, and a stepped piston (15) to enhance the pressure. The inlet pressure controls the servo inlet valve, which is normally closed to conserve hydraulic pressure when no braking is required and when the vehicle is parked. The stepped piston provides an additional pressure increase even without the pumped servo pressure.
