A check valve for use in a Y-site in an infusion system comprises a fixation portion configured to retain the check valve in a common lumen of the Y-site, and a flap member extending axially from the fixation portion. The flap member has an outer surface, at least a portion of which is configured to engage an internal wall of the common lumen overlying an inlet from a main lumen. In one embodiment, the flap member comprises a pocket, which surrounds surrounding the hole and any burr extending from the wall adjacent the hole. In some embodiments, the flap member is resiliently biased towards a sealed position, and can flex to allow fluid flow from the main lumen to the common lumen.