A braking device for a motor vehicle comprises a master cylinder (200) and a pneumatic booster (100). The pneumatic booster includes a rigid casing (10) divided by at least one moving partition (16) into at least two chambers (12, 14). A main hydraulic piston (30) of the master cylinder (200) includes a reaction piston (34) which slides in a hollow moving cylinder (32). The reaction piston includes a normally open two-way valve (94, 95) which can interrupt communication between an interior (V) of the master cylinder and the interior (35) of the moving cylinder (32); a first part (50) which abuts against the moving cylinder (32), in the rest position; and a second part (52) which abuts against the first part (50), in the rest position. A stepped third piston (82) slides inside a stepped bore (84, 85) of the moving cylinder (32) and abuts against the first part (50), in the rest position. The third piston (82) includes a central opening (90) allowing the interior (35) of the moving cylinder (32) communicate with the interior volume of the master cylinder. The diameter of the front portion (84) of the stepped bore is smaller than the diameter of the rear portion (85) of the stepped bore. |