Abstract: A servo-motor including control valve and reaction means of novel design is provided which has improved operation characteristics with a minimum of hysteresis, enabling the driver to lightly operate a brake or other associated system while accurately feeling the reaction therefrom. The control valve means precludes any loss of vacuum from a first power chamber as may otherwise occur when a second power chamber is placed into communication with the atmosphere and, eliminating the need for sealing means as arranged around an input member extending through a movable wall of the servo-motor, minimizes the resistance to axial sliding movement of the input member. The motor structure is simplified and includes a minimum number of component parts.