The invention relates to a brake-booster (10) of a vehicle, comprising a casing (12) and a diaphragm (14) which divides in a sealed manner the interior of the casing into a front chamber (16) and a back chamber (18), a hollow piston (20) movable in the casing receiving an input member (24) adapted to be connected to a brake pedal, an output assembly comprising a lid (30) and an output rod (32) adapted to be connected to a piston (34) of a master cylinder (36), a reaction disk (28) in a deformable material arranged between the hollow piston (20) and the lid (30) so as to receive an image of the output force of the booster (10) when it is put into operation in order to cause a deformation of the reaction disk (28) tending to push back the input member (24) as a function of the image. According to the invention, a sealed fluid bearing (40) connected to a valve, which is connected to a fluid sensor (42) of the load of the vehicle, is interposed between the output rod (32) and the lid (30), the bearing (40) resting on the hollow piston (20) so as to modulate the image as a function of the load of the vehicle.