Abstract: A brake booster comprises a housing; a diaphragm sealingly mounted inside the housing to define separate chambers inside the housing; a piston having a longitudinal axis (A), the piston being secured to the diaphragm, extending out of the housing, and being movable along the axis relative to the housing; a reaction member mounted in the piston; an input member extending into the piston and engageable with one side of the reaction member; a disc engageable with the other side of the reaction member; a Belleville spring mounted on the disc; an annular member positioned radially outwards of the disc and engageable with the other side of the reaction member; an output member engaging the Belleville spring, the Belleville spring biasing the output member away from the reaction disc in the rest position of the booster such that only the disc acts on the output member below a predetermined input force (F1) on the input member, and such that at or above the predetermined input force the bias of the Belleville spring