A valve unit for producing pulsed delivery of a fluid from a supply to a point of use. The valve unit consists of: a valve housing defining a main fluid chamber and inlet and outlet openings communicating with the main fluid chamber; a piston movable between first and second positions within the main fluid chamber for blocking incoming fluid flow from the inlet opening through the main fluid chamber to the outlet opening with the piston in its first position and for allowing free communication of incoming fluid flow from the inlet opening through the main fluid chamber to the outlet opening with the piston means in its second position; structure for repetitively moving the piston back and forth between its first and second positions in response to a fluid being supplied under pressure at the inlet opening, there being a charge of fluid discharged through the outlet opening in the time interval in which the piston moves out of its first position, into its second position, and back to its first position; and bleeding structure for communicating fluid from the inlet opening to a location downstream of the piston with the piston in its first position.