A valve is disclosed suitable for use in ink marking systems. The valve has an output port, a supply port, and a control port. The control port communicates with a chamber disposed above a flexible diaphragm while the output port communicates with a reservoir to be periodically supplied with fluid. The diaphragm flexes between an open position permitting the supply port to communicate with the output port and a closed position blocking such communication as a function of the pressure differential across the diaphragm created by pressure changes at the central port.