A brake booster includes a housing, a partition adapted to divide the interior of the housing into two chambers, and two diaphragms adapted to divide the two chambers into constant and variable pressure chambers. A control valve includes a valve body carried by the diaphragms, and a plunger slidably received in the valve body. The control valve is operatively connected to the variable pressure chambers to develop a pressure differential across the diaphragms to cause the diaphragms to move in response to actuation of an input rod. A reaction disk is disposed between an output rod and the valve body to transmit the reaction of movement of the output rod to the input rod. An adjusting mechanism is disposed between the plunger and the reaction disk and compressible so as to develop a greater degree of pressure differential when a force applied by the input rod exceeds a predetermined level. The reaction disk is caused to make contact with the contact disk and held out of contact with the spring and the plunger when the plunger is actuated.