Abstract: A high-pressure valve comprising a piston whose upper surface is acted upon intermittently by a medium, such as air, to control vertical movement of the piston, the valve including a connection of the piston to a needle valve. The needle valve coacts with a valve seat and communicates with the inlet to a liquid delivery line through which liquid is delivered to the needle valve via a high-pressure pump and leaves through a nozzle orifice. Also provided is a rotatable device for mechanically forcing the piston to a lower position in which the needle valve is brought into abutment with the valve seat.