Each hydraulic power pack has a 3/4"UNF industry common size 8 valve cavity which is located between the pressure line and the tank. Two main types, normally closed and normally open can be fitted directly into the hydraulic power pack central manifold. Both types are poppet style so have a free flow in one direction and operate as a check valve in the other condition.
Normally closed valves are available with a manual override in case of power loss. Typically used for tipper truck and tailifts that have a single acting circuit.
Normally open valves are typically used to "unload" the pump flow to tank until it is required. Various solenoid coils are available in either 18 or 30Watt.