Abstract: An hydraulic fluid actuated power assisted steering system has an open center valve 4 adjustable in response to a steering input to control fluid flow to a power assistance ram 6. A by-pass valve 8 communicates between a pressure line 3 and a return line 7 of the valve 4. The by-pass valve has a control port 11 which is adjustable by a valve member 9 spring biased at 10 to normally open the control port 11 so that fluid flow from line 3 to line 7 by way of port 11 alleviates the generation of noise in the valve 4 and back pressure in the line 3. The valve member 9 is responsive to a predetermined fluid pressure differential developing between the line 3 and 7 when the valve 4 is operated so that the member 9 is displaced to close port 11 and direct greater fluid flow from pump 2 to the ram 6.