A pneumatic booster utilizing first and second sources of air pressure and having a rigid casing (1) divided by a leaktight moving partition (2) into at least first and second working chambers (T1,T2). The moving partition (2) being urged by a pressure difference between the pressures supplied to the first and second working chambers (T1,T2) brought about by actuation of a three-way valve (5). The three-way valve (5) being retained in a pneumatic piston (4) carried by the moving partition (2). The moving partition (2) having a rigid skirt (3) including first and second walls (31,32) to define a fixed volume (V). The fixed volume being split into first and second reaction chambers (R1,R2) by a divider (11). The divider (11) constituting, within the booster, a reaction member intended to oppose an input force for actuating the three-way (5) to actuate the booster and develop an output force for the booster.