 Pressure cylinders containing a movable separator member between a gas and a liquid, said moveable member causing the actuation of directional control valve or valves.In circuit after a pump / or pumps, that is / are followed by valved side branch, and beyond a check valve / or valves, so that this device can only discharge liquid volume by a pressure fall of the system.Obviating use of pump drive energy unless a small pressure fall signals a system requirement for flow.