These unloading valves are used to unload pumps, to tank when pressure in a separate part of the circuit reaches a pre-set level. The valves will close, causing the circuit to reload, when the pressure drops to approximately 85% of the unload pressure. The most common application is to maintain pressure in an accumulator to be used in emergency to operate essential hydraulic functions (eg. a brake circuit). The PUL valve has a drain port to ensure correct valve function while allowing the bypassed oil to be used for auxiliary circuits.

Inlet pressure is seen on the nose of the valve and system pressure (downstream of the system check valve) operates on the system pilot port. When pressure rises to the valve setting, the relief section opens and the system pressure acts on the pilot piston to hold the valve in the open position. The ratio between the pilot piston diameter and the seat diameter to the relief valve pilot section ensures that the valve will be maintained in the fully open position until the system pressure drops to approximately 85% of the unload pressure.

Valve bodies are supplied as line bodies for the use in accumulator circuits. These include a built in check valve which provides with an internal connection to the valve pilot port.