Fourth Servo Assembly
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
Spring force actuates the fourth servo piston. This force returns the piston to its band released position. This movement forces fourth band fluid from the other side of the piston. The fluid is forced back through its circuit to the 1-2 shift valve.