chamber 5 so that the pressure in chamber 7 is still equal to the pressure on the downstream side of the main valve 1 2 and the resulting force acting on piston 4 and valve member 2 is consequently still directed towards the left so that the valve remains open it will be understood that if the annular area of piston 4 facing chamber 5 and the area of the end surface of valve member 2 facing chamber 6 are each equal to one half of the cross sectional area of chamber 7 the magnitudes of the fluid forces which urge the piston and the valve member towards the left are independent of the flow direction of the fluid with increasing flow rate the dynamic pressure acting on the valve member increases so that the valve member is urged towards the left and the flow area between this member and the valve seat increases while inversely the area decreases with decreasing flow rate by a suitable design of the valve the flow area will be able to adjust itself according to the flow rate in such a way that the pressure drop across the seat is by and large independent of the flow rate if the valve is used in a hydraulic system including a variable delivery pump it is possible to achieve a very rapid switching of the valve by combining an order signal to close or open the valve respectively with a reduction of increase respectively of the delivery rate of the pump so that the change in the pump delivery promotes the desired displacement of the valve member in case the current supply to the control circuit.