A diaphragm operated valve having the diaphragm attached at its lower end to a valve closure plug, and movable therewith, and attached at its upper end to a portion of valve body that forms the pressure cavity. The diaphragm has a stepped portion between its upper and lower ends which folds back on itself when the valve opens. The valve body has shoulder portions that support the diaphragm when the valve is closed to avoid stressing the diaphragm material due to pressure in the pressure chamber. This allows the outside diameter of the diaphragm to be minimized, and eliminates the need for fabric reinforcement. A filter screen is positioned flush with the surface of the valve plug so that high velocity flow across the screen while the valve is opening and closing removes accumulated dirt thereby making the screen self cleaning.