A pneumatic brake booster 1 comprising: a sealed structure 3 comprising a rigid casing 11 forming a funnel 19 furnished with an opening and a movable wall 13 dividing the sealed structure 3 into a low-pressure chamber 25 and a working chamber 27, a pneumatic piston 5 comprising a boss 31 fixedly attached to the movable wall 13 and mounted so as to slide in the funnel opening, a valve element 33 housed in the boss 31 and a seal 35 positioned between the boss 31 and the funnel 19, an input rod 7 and an output rod 9 coupled on either side of the pneumatic piston 5, and a return spring 37 to return the pneumatic piston 5 to a rest position, the return spring 37 being placed in the low-pressure chamber 25 between an internal surface 45 of the rigid casing and the pneumatic piston 5, and a force sensor 39 placed in the low-pressure chamber 25 and subjected to the pressure variations of the return spring 37.