Abstract: A differential servo-motor mechanism in which manual force is supplemented by power assistance, comprising, a housing, a pressure responsive wall in the housing, valve means associated with said wall and being movable relative thereto to control the differential pressure acting on said wall, an output member, a resilient element for normally urging the wall toward its rest position, and reaction means characterized by a plurality of reaction elements arranged to provide the reaction means in the form of levers each having first, second, third and fourth points spaced radially inward from each other and engageable with the resilient element, the wall, the output member and the valve means respectively.