Number 2 Ball Check Valve (372)
WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Located in the case cover (400), 2nd fluid seats the ball check valve and is forced through an orifice into the 2nd clutch passage to apply the 2nd clutch.