According to the diagram, the same pin of the pushbutton controller provides power to both the center and defrost switchover valves. From what I've seen of the ACC system, they usually switch the ground side on/off. A meter at the switchover valves should verify the electrical side.
However, as Brian mentioned, the vacuum side is more suspect. There is a four-way connection that ties the defrost and center pods together. If the defrost pod is leaking, the center pod will not get sufficient vacuum (due to an orifice in the line) to close as well. There are two vacuum sections to the defrost pods and you will probably find one section is leaking. It is best to replace the entire pod and it is one the easiest to access in the entire ACC system.
Here is a link to the vacuum diagram:
http://www.startekinfo.com/StarTek/outside/12253/program/Climate/83-604.pdf
Scroll down to "cooling".