Abstract: A pressure operated valve is arranged so that it switches over rapidly between two opposite end positions. The valve has a pressure chamber closed by a diaphragm. Between the diaphragm and the valve member, there is a snap-action spring which continues to hold the valve member in one end position during a first phase of diaphragm movement, and at a preset point snaps over to a condition where it accelerates the valve member into its opposite end position, and holds it there. A biasing spring may oppose the force exerted on the diaphragm by pressure (normally vacuum) in the pressure chamber. The valve has use in EGR systems, as well as engine decelerating systems.