space 12 after having passed the gap between the front face 7 of the right piston portion 4r and the valve seat 8 the oil flows through the tube 9 and the channel 15 back to the oil pump if the control air pressure at the outlet of the control air source st increases air is directed through the channel 14 to the large left piston portion 4l of the twin piston 4 because of this the twin piston 4 moves to the right in the drawing and throttles the valve seat 8 now the pressure in the pressure space 12 increases until the forces acting upon the smaller right piston portion 4r and directed to the left in the drawing are equal to the forces directed toward the right side and generated by the control air acting upon the large left piston portion 4l the area ratio of the choke valve 11 to the throttling gap between the front face 7 and the valve seat 8 always assumes a value where the product from the large piston surface and the control air pressure is equal to the product from the surface of the small piston and the hydraulic adjusting pressure so that the required proportionality between a control and adjustment signal is obtained naturally other elements for instance a tubular.