A power braking device including a pneumatic brake actuator (1) combined with a master cylinder (2) and provided with an electrically operated automatic control valve (26). Said actuator operates according to a mixed reaction principle and includes a movable pneumatic piston (5) supporting a control valve (7) actuated by a rod (8), as well as a partition (4) which is moved by a pressure differential and slidable relative to the pneumatic piston (5). Said device provides automatic braking with limited means and without disrupting the normal operation of the actuator.