2-Way Check Valve
WARNING: This page is about a different car, the 1995 Mazda RX-7. However, it is still accessible from the selected car via links, so may be relevant.
Remove check valve. Connect a vacuum pump to outlet end of check valve (see arrow on valve). Air should flow with about 1.7 in. Hg vacuum applied. Connect vacuum pump to other end of check valve. Air should also flow with about 1.00-1.46 in. Hg vacuum applied. Replace valve as necessary.