A check valve assembly includes a first port fitting and a second port fitting with a base secured therebetween. The base has an annular sleeve encircling a seat, the seat having a central mounting hole and one or more flow channels passing therethrough. An umbrella valve includes a flexible sealing disk having an outer surface and an opposing inner surface, a mounting stem extending from the inner surface and projecting into the mounting hole of the base, the sealing disk being movable between a first position wherein at least a portion of the sealing disk sits on the seat so as to cover the one or more flow channels and a second position wherein the sealing disk is resiliently flexed so as to at least partially uncover the one or more flow channels. A retention plate is disposed between first port fitting and the seat of the base so that the retention plate sits against the outer surface of the sealing disk.