In one embodiment, a constant-flow valve assembly is provided that comprises a first fluid passageway carrying fluid at a first fluid pressure, a piston chamber, and a second fluid passageway connected to the chamber and carrying fluid at a second fluid pressure. A third fluid passageway is configured to carry fluid at a third fluid pressure. An adjustable valve member is provided between the chamber and the third passageway to provide constant fluid flow to the third passageway. An adjustable restrictor assembly is positioned between the first and second fluid passageways. An inlet portion of the restrictor assembly receives fluid at the first fluid pressure and directs the fluid to a restrictor. An outlet portion receives fluid from the restrictor and directs the fluid to the second fluid passageway at the second fluid pressure. The restrictor is movable to adjust the position of entry and exit portions relative to the inlet and outlet portions to adjust a fluid flow rate through fluid pathway to the second fluid passageway, thereby adjusting the flow rate through the valve assembly.