A liquid delivery system comprises a tank containing a supply of a first liquid. A positive displacement pump operates to withdraw the first liquid from the tank via a suction conduit and to deliver a pressurized flow of the first liquid via a delivery conduit. An adjustable metering orifice in the delivery conduit serves to control the flow of the first 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 first liquid exceeding a threshold level, and is operative when open to deliver the first 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 second valve in the bypass conduit is adjustable to accommodate a controlled return flow of the first liquid from the delivery conduit to the tank, thereby modulating the pressure of the first liquid being delivered to the first valve.給液システムであって、液補給用のタンクと、タンク内の液を吸込路経由で吸い込みその加圧流を給液路上に送り出す容積移送式のポンプと、給液路内にあり給液路内液流の調節手段として働くメータリングオリフィスと、給液路内のポンプ・メータリングオリフィス間部位にあり、液圧がしきい値レベルを超過すると開弁してメータリングオリフィスに対し略一定の圧力で給液する常閉型のバルブと、タンクに通ずる一方でタンク・バルブ間部位にて給液路に通ずるバイパス路と、を備える。
