Ill - DESCRIPTION Two main-units comprise the hydraulic part of the steering. These are the piston/ cylinder. and the Control Valves with rotating union. ]. Piston/ cylinder unit : This is a double-acting piston connected to the rack. 2. Valves with Rotating Union : - Two pressure-control slide-valves (one for each side of the piston ) are connected to the steering by way of a coupling fork.