Brake booster with a double booster ratio for motor vehicles

The braking amplifier with double ratio amplification comprises a thrust set (71) comprising a reaction rod (82) connected to the brake master cylinder. The rod can be axially displaced under the control of a plunger (22) between a first braking state corresponding to a first braking force amplification ratio and a second braking state corresponding to a second ratio. A deformable reaction disc (96) is interposed between a piston (70) and the plunger and the reaction rod. An annular cavity (104) absorbs the deformation of the reaction disc.