In its simplest form, the valve comprises a body with an inlet connector and an outlet connector intended to be connected to a pressure source and a pneumatic circuit respectively. In the body, a valve comprising a seat and an obturator controls communication between the connectors. A control piston is sealingly slidably mounted in the body and, in its working travel, can cause controlled movement of the obturator away from the seat to cause controlled opening of the valve. According to the invention, the seat of the valve is formed in a member mounted in the body for movement in the direction of displacement of the obturator. A member is operatively associated with the piston and intended to cause a displacement of the seat relative to the body together with and in contact with the obturator when the control piston passes beyond a predetermined position relative to the body in its working travel.
