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.