A changeover valve unit for a power-assisted steering system includes input and output shafts arranged for relative rotation on aligned axes, a torsion bar interconnecting the shafts, a spool valve element surrounding the input shaft and arranged to be axially displaced in response to relative rotation of the shafts for controlling the supply of pressurized fluid to a hydraulic cylinder associated with the steering system, and a displacement mechanism for the spool valve element. The valve unit further includes an annular retainer member engaged with one end of the spool valve element to form an oil chamber, a compression spring assembled within the oil chamber to bias the spool valve element toward the displacement mechanism, and an annular orifice formed around the input shaft by means of the retainer member to permit the flow of fluid between the oil chamber and the interior of the spool valve element.