Abstract: In combination with a hydraulic step motor comprising a servo-valve having a rotary valve member and a housing surrounding this member. The rotary valve member is provided with two fluid manifolds conjointly movable therewith. Two main conduits communicating with a source of fluid under pressure provide passage for the fluid to the servo-valve and to the fluid manifolds.A drivable shaft of the step motor is connected to the servo-valve rotary member to drive same. The rotary valve member comprises a plurality of primary positioning projections and of secondary sealing projections alternately spaced from one another and uniformly distributed around the periphery of the rotary valve member. Between the adjacent projection gaps are formed which are alternatively connected with one and the other of the fluid manifolds. Fluid channels provided in the wall of the housing open towards the periphery of the rotary valve member.