Pneumatic brake servo with improved seals

The pneumatic servomotor has a housing (101) made from a bowl (105) with a rim (153) and a cover (103), with its inner volume (107) divided into a variable pressure working chamber (109) and a low pressure chamber (111) by an inner sliding and sealed skirt (113) with a central aperture (115) for a pneumatic piston (117) with a three-way valve (123). The two chambers are sealed from one another by inner (156) and outer (157) seals, the outer seal being fitted in a fold in the edge of the skirt enclosing a low-pressure cavity.