Fluid-flow valves

In a valve, means defining a multi-part casing, said casing comprising a first casing part, a second casing part, and at least two fluid flow connections, a membrane located in the casing between the first and second casing parts and acting as a dividing wall between these parts, a switching member mounted in the casing, said switching member having at least one curved switching surface bearing against the membrane, and first and second curved surface portions adjacent the switching surface, said first and second surface portions being curved in the opposite direction to the switching surface and said switching member being movable into a plurality of switching positions to provide a selected communication between, or closure of, the connections, means for moving the switching member between the switching positions, and means defining a projection at the end of the first curved surface portion to prevent the membrane from bulging when the membrane is subjected to pressure and cannot bear against the said first curved surface portion.