Fluid flow control valve means for servo systems operate in response to an input load to control fluid flow to a fluid pressure operated device, which produces an output load. A valve means typically comprises inlet and exhaust valves 10 and 15 operated by the input load and a feedback from the output load, which may be too slow to operate the valves 10, 15 successfully. The valve (e.g. 15) therefore has a valve member 6 responsive to the input load, which is biassed by resilient means 16 towards a valve seat 8, which is movable in response to the pressure differential across it such that for at least one mode of operation of the valve, the seat 8 moves in response to a change in the pressure differential to counter the effect of a change in input load. This effect is known as negative feedback. The valve means is preferably arranged so that the effect operates for opening of both inlet and exhaust valves 10, 15. The resilient means 13 may comprise a spring, or a flexible diaphragm 12, 18.