Pneumatic valves (COPM)
In this type of valves, the perfect closing is ensured by the adjustable pressure of the spring. For the operation and adjustment of the flow, in the cylinder is required a pneumatic backpressure (or, if you prefer, a hydraulic backpressure). In any case, if the backpressure is not available, the valve can be open by the manual control.
* The valve works even with the vacuum thanks to the obliged opening.