Most preferably, the valve member comprises at least one substantially solid plate, preferably a disk, having at least one hole therein of section corresponding to the section of the chamber, the plate being rotatably mounted about an axis perpendicular to the section of the chamber at the valve zone so that a portion of the plate extends through said section of the chamber, alignment of the or each hole with the valve-zone of the chamber providing an opening through which the or each piston member can pass to provide the open condition of the valve, and alignment of a solid portion of the plate with the valve-zone providing the closed condition of the valve.
Preferably, fluid is admitted to or exhausted from the chamber by means of one or more ducts disposed around the chamber, the ducts being positioned so that movement of the or each piston member within the chamber selectively exposes volumes of fluid disposed between the piston member and the valve member to a respective duct and selectively inhibits fluid communication between a respective volume and the respective duct.
