#1 Checkball (Low/PRN)
WARNING: This page is about a different car, the 2004 Pontiac Sunfire, 2004 Chevrolet Classic, and 2004 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
Low fluid pressure seats the #1 checkball against the PRN fluid circuit and this pressure fills the Low/PRN fluid circuit. Low/PRN fluid is routed to the 1-2 shift valve.