Reverse Input Clutch Piston
WARNING: This page is about a different car, the 2009 GMC Yukon and 2009 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.
Reverse input fluid pressure exhausts from the piston, through the boost valve, past the #3 checkball and to the manual valve. With the reverse input fluid exhausted, the reverse input clutch plates are released and the transmission is in Neutral.