A check valve assembly (10A, 10B, 10C, 10D, 10E) comprising: a first port fitting having a tubular stem with a passage extending therethrough; a second port fitting having a tubular stem with a passage extending therethrough; a base disposed between the first port fitting and the second port fitting, the base comprising an annular sleeve encircling a seat, the seat having a first side and an opposing second side with a central mounting hole passing therethrough and one or more flow channels passing therethrough; an umbrella valve (90A, 90B) comprising a flexible sealing disk having an outer surface and an opposing inner surface, a mounting stem extending from the inner surface of the sealing disk and projecting into the mounting hole of the base, an enlarged head being formed on the free end of the mounting stem so as to be disposed between the seat of the base and the second port fitting, the enlarged head having a diameter larger than a minimum diameter of the mounting hole of the seat, the sealing disk being movable between a first position wherein at least a portion of the sealing disk sits on the seat of the base 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; and a retention plate (92A, 92B, 92C) disposed between first port fitting and the seat of the base so that the retention plate sits against or adjacent to the outer surface of the sealing disk, the retention plate comprising a plate body having a first side and an opposing second side, the first side of the plate body sitting against or adjacent to the outer surface of the sealing disk, the retention plate further comprising a plurality of legs projecting from second side of the plate body so that the plurality of legs space the second side of the plate body away from the first port fitting. a first port fitting having a tubular stem with a passage extending therethrough; a second port fitting having a tubular stem with a passage extending therethrough; a base disposed between the first port fitting and the second port fitting, the base comprising an annular sleeve encircling a seat, the seat having a first side and an opposing second side with a central mounting hole passing therethrough and one or more flow channels passing therethrough; an umbrella valve (90A, 90B) comprising a flexible sealing disk having an outer surface and an opposing inner surface, a mounting stem extending from the inner surface of the sealing disk and projecting into the mounting hole of the base, an enlarged head being formed on the free end of the mounting stem so as to be disposed between the seat of the base and the second port fitting, the enlarged head having a diameter larger than a minimum diameter of the mounting hole of the seat, the sealing disk being movable between a first position wherein at least a portion of the sealing disk sits on the seat of the base 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; and a retention plate (92A, 92B, 92C) disposed between first port fitting and the seat of the base so that the retention plate sits against or adjacent to the outer surface of the sealing disk, the retention plate comprising a plate body having a first side and an opposing second side, the first side of the plate body sitting against or adjacent to the outer surface of the sealing disk, the retention plate further comprising a plurality of legs projecting from second side of the plate body so that the plurality of legs space the second side of the plate body away from the first port fitting.