Power braking device comprising a pneumatic brake servo (1) associated with a master cylinder (2). The servo comprises a movable pneumatic piston (5) carrying a valve (7) and operates on a hydraulic reaction principle, the hydraulic piston (11) of the master cylinder comprising a hollow movable cylinder (13) receiving a boosting force, and a secondary hydraulic piston receiving an input force, both these forces constituing the total actuating force. A helical spring (22) between the secondary hydraulic piston and the movable cylinder exerts an adjustable resilient force in order to adjust the non-zero value of the actuating force.