The invention relates to a control valve assembly for a hydrostatic control system such as power steering assembly. A common control valve assembly of this type has relatively rotatable inner and outer elements connected respectively to a vehicle steering wheel and a servomotor. Paired holes in the inner and outer element permit the flow of fluid therethrough when the unit is in its neutral position with the paired holes in respective registration. In hydrostatic steering system of this type impact is sometimes applied to the steering wheel when the working motor connected to the steered wheels is suddenly loaded, e.g. when the wheels strike a stone. Such impacts on the steering wheel are avoided by providing a one-way valve in a section of the working path through which fluid flows to the working motor. Pressure regulating valve means, excess pressure valve means and emergency control valve means are associated with the one-way valve assembly.