The invention relates to a tandem pneumatic brake booster (1) for a vehicle. The objective is to prevent a blocking of a booster thrust rod (4) in the maximum travel position. The blocking appears following a sealed adhesion of a first plate skirt (12) on a seal (22) of the tandem booster. This seal blocks a vacuum in a first front chamber (5). To prevent this blocking, provision is made to crenelate a first lip (23) pressed against the pneumatic piston (3) of a seal. Merlons prevent a sealed adhesion of the plate skirt (12) over the whole seal (22). This absence of adhesion creates a leakage of air to the first front chamber (5) and allows a return to a rest position.
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