Dual hydrostatic vehicle steering system

A vehicle including a steering system for moving a pair of wheels supported by a wheel support structure, relative to a vehicle structure to achieve steering of the vehicle whilst the vehicle is travelling, the system including a pressure source for providing a supply of pressurized fluid, to first and second steering valves, and first and second fluid operated actuators each of which includes an actuating member, the steering valves each being operator controlled to provide the pressurized fluid, at least when the steering system is in a first operating mode, from the first steering valve to the first fluid operated actuator and from the second steering valve to the second fluid operated actuator to achieve steering, and when the steering system is operating in a second operating mode, one only of the first and second steering valves providing pressurized fluid from the pressure source to a respective one only of the fluid operated actuators to achieve steering, and wherein there is a valve structure between the steering valves and the pressure source which includes a valve member which is moveable by the upstream pressurized fluid where available, to provide fluid paths for pressurized fluid from each of the steering valves to respective fluid operated actuators, but in the absence of sufficient fluid pressure, the valve member moves to a position to establish the fluid path from the one steering valve to each of the first and second fluid operated actuators.