Mecline VRT3-310EZ Unloader with 4,500 max PSI, 10.5 max GPM and 3/8"F outlet, black spring and adjustment knob.  The unloader valve diverts water flow into bypass when the trigger gun valve is closed. Unloaders may be designed to respond to either an increase in pressure or a change in flow. The unloader valve may also be used for variable pressure adjustment.