The spray gun and unloader make up a two-part valve system that directs water into the bypass and back to the pump or float tank. The spray gun shuts off the flow of water causing the unloader to re-circulate the water back into the inlet side of the pump. This is called sending the water to bypass. Pressure actuated unloaders are opened by the increase in pressure occurring when the valve in the spray gun is closed. Flow actuated valves send the water into bypass when flow drops.
This is a pressure actuated model.
Unloader valve for 1991 Graco 
