Abstract: This invention comprises an improved hydraulically operated flush valve for toilet flush tanks and similar devices. Its main components are a cylinder and piston assembly positioned with its longitudinal axis directly above and vertically aligned with the vertical axis of the outlet or flush tube of the flush tank. The moveable piston within the fixed cylinder acts as the moving and sealing element of the flush valve. By means of an inner valve connected to the flush tank operating lever hydraulic force is caused to lift the piston thereby opening the flush valve. There is no direct mechanical connection between the operating arm and the flush valve, and there are no chains or hinges. There is a positive permanent flush valve alignment and a minimum of friction. In addition, the amount of water used per flushing action can be regulated to prevent water and energy wastage.