Abstract: A vacuum security valve includes a buffer volume in a movable piston. The movable piston has two orifices communicating with the buffer volume, the first orifice communicating with a clearance volume above the piston. A flexible diaphragm is connected to the piston for separating the clearance volume from the remainder of the interior of the valve body. Another orifice, disposed to communicate with the remainder of the interior of the valve body rather than with the clearance volume, is provided in the piston wall, and also communicates with the buffer volume. A solenoid operated valve, which is normally in the opened position, communicates through the valve body with the clearance volume, and also communicates with a vent. The security valve is connected at one port to a vacuum system which is to be protected upon power failure of a vacuum pump. The other port of the security valve is connected to the lines leading to the vacuum pump.