An adapter assembly (22) including a manifold (30) and a valve assembly. The valve assembly includes a seat (202) and a valve body (200) having a circular base (210) and a wall (212). The wall (212) extends from a trailing side (218) of the base (210) to form a dome-like shape terminating at an end at which a slit (230) is formed, with the wall (212) defining opposing sealing edges (240) at the slit (230). The seat (202). has an upper circumferential surface (250) and a lower, circumferential surface (252). The upper surface (250) engages a leading side (216) of the- base, (210) whereas the lower surface (252) engages a trailing side(218). At least one of the upper (250) and lower surfaces (252) forms a segment of increased height (266,286). Upon final assembly, the valve body (200) is disposed across a passageway (44) of the manifold (30), with the slit (230) providing a selectively openable path. A force imparted by the segment of increased height (266,286) flexes the base (21O) and biases the sealing edges (240) into engagement.
