Abstract: This brake booster comprises an enclosure (40), a diaphragm (42) dividing the enclosure into a front chamber (46) connected to a source of vacuum and a rear chamber (48), a piston (44) that can move axially in the enclosure and on which the diaphragm is mounted, a plunger (50) mounted so that it can move axially in the piston between a forward unstable braking position and a retreated stable position of rest, and a valve (54) intended to collaborate with seats (56, 58) on the piston and on the plunger. The or each seat (56) on the piston (44) is formed on the edge of a passage (60, 62) allowing the said chambers (46, 48) to communicate.