Number 5 Ball Check Valve (373)
WARNING: This page does not describe the selected car, but rather 13 other vehicles, including the 2006 Saturn Relay, 2006 Pontiac Montana SV6, 2006 Pontiac Grand Prix, 2006 Pontiac G6, and 2006 Chevrolet Uplander. However, it is still accessible from the selected car via links, so may be relevant.
Located in the control valve body, (300), it blocks the reverse servo feed passage forcing reverse fluid through an orifice in the spacer plate (370) into the reverse servo passage. When the manual valve (404) is moved out of reverse, the ball check valve unseats allowing reverse servo fluid to exhaust through the ball seat instead of through the orifice.