The VCB has a fine micro filter on the suction side and on the outlet side an oil and oil mist separator to return the oil to the oil circulation system. A fan between the pump housing and the motor provides intensive air cooling. The fan is located in a fan cover that guards against it being touched. An integral non-return valve prevents the evacuated system from being ventilated after the pump has stopped. If the machine has been idle for more than two minutes the connected pipe should be ventilated to atmospheric pressure. A gas ballast valve prevents water vapor condensation from getting into the inside of the pump when it is sucking in small amounts of steam and the pump is at operating temperature.