Abstract: An unloader for a high pressure hydraulic system, as a high pressure spraying system, has a pressure inlet for connection with a pump, a pressure outlet for connection with a load and a bypass outlet with a bypass valve. A piston moves the bypass valve between open and closed positions. The flow of liquid through the unloader from the inlet to the pressure outlet and to the hydraulic load establishes a pressure differential across the piston to close the bypass valve. When no flow to the load occurs, there is no differential pressure across the piston and the bypass valve is opened.