Brake-booster with hydraulic reaction and adjustable kick

A brake-booster device for a motor vehicle, comprising a master cylinder having with a main hydraulic piston designed to receive an actuation force composed of an input force and of a boost force both acting in an axial direction, and a pneumatic booster controllable by application of said input force on a control rod controlling the opening of a valve, so as to exert said actuation force on the main hydraulic piston, the booster comprising a rigid casing separated in leaktight manner into two chambers by means at least of a movable partition capable of being acted upon by a pressure differential between the two chambers resulting from the opening of the valve and of driving a pneumatic piston, movable with respect to the casing, bearing said valve and contributing at least to transmission of said boost force, characterized in that the main hydraulic piston of the master cylinder comprises a hollow, movable cylinder communicating with the master cylinder, receiving at least part of the boost force, and inside which slides, in leaktight manner and in the axial direction, a secondary hydraulic piston capable of receiving at least said input force, and in that resilient means, exerting an adjustable resilient force between said secondary hydraulic piston and said movable cylinder, is disposed inside the movable cylinder and pushes said secondary hydraulic piston in the direction of the master cylinder, the adjustment of said resilient force enabling adjustment of a non-zero minimal value of the actuation force.