Our problem relates to a master cylinder with a U-shaped seal. Our goal is to avoid defective bleeding of hydraulic braking systems. The defects arise out of the fact that the vacuum causes the lips of the seal (21) to stick together and to stick against the wall (41). To achieve this, reliefs (42) will be created on one wall (44) of one lip (28) facing the other lip (27) and on the edge face (50) of this same lip. These reliefs will prevent the lips from sticking tightly to one another and to the wall facing the edge face of the lips. This lack of sticking will prevent a vacuum from being created between the two lips and between the edge face of the lips and the wall (41). As a result, the seal will be able to return to a position in which bleeding can be performed effectively.