A dual hydraulic circuit arrangement is provided each comprising essential elements of an independent booster steering system, each having valving mechanisms for steering control wherein the valving mechanisms pressurize and exhaust the power cylinder of the respective circuit. The valving mechanisms are operable in a sequential order so that initially and ordinarily one power cylinder effects steering, but in the event of failure or very heavy requirement the second power cylinder is capable of operating the steering mechanism or of assisting in such operation. It is contemplated that the pump of the initially operative circuit is engine driven and the pump of the other or emergency circuit is driven by the vehicle wheels. Thus, in the event of vehicle engine failure, the emergency power circuit can effect steering as long as the vehicle is moving. On the other hand, when heavy steering load is encountered, as in parking, both circuits are operative.