The unloader valve is a pressure washer's most important safety feature. The unloader valve allows pressure to be released when the trigger is not engaged, preventing the machine from building up too much pressure, is designed to respond to an increase in pressure or a change in water flow.
The unloader valve is located between the pump and the trigger, if the unloader valve is not working properly, the pressure washer can be very dangerous. Always check the unloader valve before using a pressure washer, and be sure to follow the instructions for its proper use.