A liquid delivery system comprises a tank containing a supply of liquid. A positive displacement pump operates to withdraw the liquid from the tank via a suction conduit and to deliver a pressurized flow of the liquid via a delivery conduit. A metering orifice in the delivery conduit serves to modulate the flow of the liquid through the delivery conduit. A normally closed first valve located in the delivery conduit between the pump and the metering orifice is opened in response to a pressure of the liquid exceeding a threshold level, and is operative when open to deliver the liquid to the metering orifice at a substantially constant pressure. A bypass conduit communicates with the tank and with the delivery conduit at a location between the pump and the first valve. A flow restrictor in the bypass conduit serves to modulate a return flow of the liquid from the delivery conduit to the tank, thereby modulating the pressure of the liquid being delivered to the first valve.
