Abstract: An apparatus for converting a hydraulic pressure signal to a mechanical motion in a way such that the amount of physical displacement of an element is directly proportional to the value of a hydraulic signal, the apparatus having a housing having a cylindrical cavity, a diaphragm within the housing dividing the cavity into a first portion and a second portion, the second portion of the cavity having walls configured as a frustocone, the base of the cone being the inner surface of the diaphragm, a shaft slidably positionable in the housing, the inner end of the shaft connecting the diaphragm so that hydraulic pressure applied to the rearward surface of the diaphragm displaces the shaft, the diaphragm contacting the conical walls as it is moved forward by increased hydraulic pressure, decreasing the effective area of the diaphragm such that the displacement of the shaft is substantially directly proportional to the pressure of the hydraulic signal.