Pneumatic booster

A pneumatic brake-booster for a vehicle comprising an engine compartment and a passenger space which are separated by an apron, the booster comprising a casing formed of a front shell and of a back shell and inside which is located a piston comprising of a back tubular part and supporting a skirt which, with the aid of an unrolling membrane, defines a first chamber connected permanently to a source of partial vacuum and a second chamber connected selectively to a one of the front chamber and to atmosphere by valve means actuated by a control rod, and the casing comprising means for securing the booster to the apron of the vehicle, the booster comprising a third chamber connected permanently to atmosphere through a filter element and selectively to the second chamber by the valve means, the third chamber being partially delimited by a bellows secured in leaktight manner to the control rod, by a back face of the back shell and by a front face of a housing secured in leaktight manner to the casing of the booster and onto which the bellows is secured in leaktight manner, the third chamber being formed between the back shell of the booster and the apron of the vehicle, wherein the housing comprises a cylindrical peripheral part whose diameter is substantially equal to that of the casing of the booster, and formed with an aperture associated with the filter element disposed in the third chamber in the immediate vicinity of the aperture, the third chamber having a volume at least equal to a minimal volume of the second chamber, and the means for securing the booster to the apron of the vehicle passing in leaktight manner through the third chamber.