#6 Reverse Shift Ball Check Valve
WARNING: This page is about a different car, the 2009 Pontiac Torrent and 2009 Chevrolet Equinox. However, it is still accessible from the selected car via links, so may be relevant.
Exhausting drive or reverse (20) fluid seats the reverse shift ball check valve against the drive (19) fluid passage, enters the reverse (22) fluid passage and continues on to an exhaust at the manual valve.