BRAKE BOOSTER DEVICE WITH EMERGENCY VALVE

A braking device, for a motor vehicle, comprising: a master cylinder (2), a primary piston (3) mounted to slide in the master cylinder, a manual-control member (4), a booster coupled to the manual-control member, an emergency assist valve (VA) comprising a reaction piston (17a) and a rapid piston (17b), and a ratio control (T) actuated by a plunger distributor (10) driven by the manual-control member (4). The reaction piston (17a) and the rapid piston (17b) form one and the same stepped piston (17) having a part (17a) of large cross section and a part (17b) of small cross section, the large-section part (17a) determining, with a first bore (18) of the primary piston (3), a variable-volume annular chamber (27). A separation/communication assembly (28, 29, 30), controlled by the displacement of the stepped piston (17), is designed so that the pressure of the liquid is exerted effectively on the large cross section (17a) of the stepped piston when the stepped piston occupies a position of rest or is to the rear of the position of rest, and on only the small cross section (17b) when the stepped piston is displaced forward relative to the main piston under emergency braking.