A power steering system includes a rotary slide valve which has a housing, an input shaft and an output shaft which can be rotated relative to each other to a limited extent. A centering piston is provided which is non-rotatably and axially movably connected with the input shaft and delimits a pressure space in the housing. The rotary slide valve further comprises a centering ring which is non-rotatably connected with the output shaft, and arm inlet to the pressure chamber. The pressure chamber has a controlled outlet. There is also proposed a method of operating such a power steering system.