A pilot operated ball cock valve for controling the height of fluid in a tank. The rotary pilot valve has an inlet port and an outlet port which are interconnected and communicate with a fluid channel which bypasses an evacuation chamber and a diaphragm valve permitting fluid to flow directly to the inlet port of the rotary pilot valve. The outlet port of the rotary valve is positioned by a predetermined fluid height in the tank into alignment with a fluid channel which transmits fluid pressure into a pressure chamber above the diaphragm. Such positioning allows fluid pressure to act upon the diaphragm which closes a discharge orifice thereby preventing flow of fluid into the tank. A decrease in fluid height in the tank induces the rotary pilot valve to rotate such that the outlet port will move out of alignment with the fluid channel leading into the pressure chamber. A lip seal containing the outlet port of the valve is then forced by fluid pressure against the well surface with which the rotary pilot valve articulates preventing the leakage of fluid through its outlet port. A fluid channel communicating with the evacuation chamber and the periphery of the rotary pilot valve allows the pressure in the pressure chamber to be relieved and the diaphragm opens permitting fluid to be evacuated into the tank until the fluid level reaches the predetermined height where the outlet port of the rotary pilot valve is realigned for transmission of fluid pressure into the chamber.