In order to assure that vacuum is available to the vacuum brake booster, the car can contain a vacuum pump either in addition to the normal intake manifold vacuum source or as stand-alone. The pump provides negative pressure when the engine cannot properly accommodate the requirement, e.g. cold start, heavy throttle, high altitude and turbo pressure.

The mechanical vacuum pump is driven by a camshaft. The pump delivers maximum vacuum irrespective of engine RPM. This means that the pump, in certain situations, delivers more negative pressure than the system uses.

A low vacuum switch controls the 12V to the relay coil in the brake vacuum assist pump. When more vacuum is needed the vacuum switch closes. The relay coil closes the relay contact and connects the electric motor to ground. The electric motor is fed with 12V from BEC. When sufficient vacuum is reached the switch opens and the pump stops.

A venturi is connected between the intake manifold and the vacuum brake booster, amplifying the vacuum. This reduces the on-time for the pump. To prevent air flowing in the wrong direction check valves are used.