I poked around in history, looking for a thread that Chris Johnson updated back in like 2010 (or earlier) with a decision tree. I didn't find it. But I did find a few other things you might want to read up on...
Air Suspension Trouble Shooting Guide (From the Star Motors Website)
Martin Werminghausen's Site This page has links to read & watch, especially the "Sherlock tests"
Threads...
The whole car drops Has several diagnostic procedures
A front valve issue Also has suggestions about how to manually fill the suspension
Overhauling Air Control Valves A great technical read, but don't do this yourself unless you are a seasoned machinist.
Rear axle maintenance and R&R While you have the rear apart, consider some of this
Leveling Valves and Diagnostics
The rear goes down first, then the front Not a lot of diagnostics, which is why I listed this one last.
Hope some of that "light reading" helps.
-CTH