It's been a while, but I thought I'd wrap this up. I appreciate everyone's suggestions; I discovered what the problem was after I gave up and bought a set of springs and removed the air suspension completely. As I was dismantling everything I discovered that the "air supply control unit" had corroded at the contacts, in spite of their being no water damage in that area at all (which, I'm told, is something you have to watch out for.) Part #13 in the diagram, can get used ones on the eBays for maybe $40. But now I don't have to worry about that system malfunctioning and eating my money ever again, so I'm ok with that.
