Abstract: Shunts (valves) (100) for controlled flow of bodily fluids, such as cerebrospinal fluid, include a valve actuating member (114) (such as a piston (120)) between inlet (108) and outlet (110) ports. The valve actuating member is biased toward a state wherein the valve is closed, and it includes a face adjacent the upstream side of the valve (and preferably isolated from the downstream side of the valve) whereby fluid pressure in the upstream side exerts an opening force on the face to urge the valve toward an open state. When the opening force on the valve exceeds the closing force by some amount, the valve actuating member moves the valve to the open state, allowing fluid flow through the fluid passage between the inlet and outlet ports.