I forget what I've posted here already... The short answer is that I took apart and inspected the duovalve, and all seemed to be well. I picked up a can of refrigerant and a fill hose and added about six ounces of freon (134a), about two ounces at a time over a long period.

I now have cool air out of all vents. It has been about six weeks since this problem cropped up, and at least a month since I added that refrigerant, and I have not had a loss of cooling capacity yet. I know freon doesn't just go away without a leak, but I have no idea about the prior owner. Since I recharged the system a bit, things are well. I'll be keeping a close eye on it during the hot Texas summer!

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI