Abstract: A back-flow prevention valve assembly is disclosed for use with a vacuum pump or the like and employs a valve element movable between positions opening or closing a passageway through the assembly. In the event of pump failure, a control valve, which is operated by fluid from the pressure side of the pump, vents ambient air behind the valve element to assist it in moving rapidly to the closed position to prevent backflow. After resumption of pumping, a controlled withdrawal of ambient air behind the valve element delays opening of the valve until the pump has created a sufficient vacuum to minimize influx of air upstream when the valve opens.