Steering Control Unite The FK or FKS valve block is bolted on the port face of the PSU directly. Its function is to protect the steering system. The hydraulic function symbol: Function of the incorporated valve Inlet safety valve: limits maximum pressure drop across the PSU to protect the steering circuit. Cylinder port shock valve: protects hoses against pressure surge created by ground forces on the cylinder axle. Anti-cavitation valve: protects steering circuit against vacuum (cavitation) conditions. Inlet check valve: prevents oil from returning through the PSU when pressure on the cylinder side is greater than the inlet side. It prevents steering wheel kick. Internal check valve: converts the PSU to a hand operated pump for limited manual steering.