A check valve for a housing with adjacent first and second apertures separated by a septum comprises a hinge pin, a first flapper, and a second flapper. The hinge pin is located downstream of the septum. The first flapper is anchored via the hinge pin, and is rotatable about the hinge pin to block the first aperture when in a closed position. The second flapper is anchored via the hinge pin, and comprises a primary flapper section and a counterbalance section. The primary flapper section is rotatable about the hinge pin to block the second aperture when in a closed position. The counterbalance section extends upstream from and substantially parallel to the primary flapper section, towards the first aperture.