The invention relates to a valve (300) comprising a base (301) in the form of a ring defining a central opening (309), as well as an upper surface (302) and a lower surface (303), both of which are planar and converge toward each other from the base (301) in the direction opposite said base, and characterized in that the upper and lower surfaces (302, 303) together form an angle (Ang) of at least 60°, and in that the free edges (304, 305) thereof define a slit (306) which is open when the valve is at rest.
