I'm working this very problem at the moment. My vacuum leak this time was the recirculation door control bellows. Once the system got through the "recirculate" phase the system worked normally. Using a mechanic's stethescope with the end open I was able to trace the "sucking" to the inside of the duct where the recirculated air is drawn in.

I have the old bellows out after cutting the sousing some and pulling "like hell". I just picked up the new one from the dealer yesterday. Hopefully by Monday my AC will be working perfectly again. (I plan to take some pictures.)

There are 4 bellows in the system with 3 design variations. (2 pull, 1 dual, 1 push) The 2 located in the air distribution box in the center are "pull" type and never have problems, (that I've heard of). The 3rd one in the center is the dual acting one that pushes as well as pulls. That one usually fails first when the seal around the actuating rod fails so it cann't push any more. I have the factory manual pages for replacing that scanned and can send them to you. There is a short cut described in the 700/900 FAQs.

The 4th bellows is located on the far end of the unit in the back and controls air recirculation. On the 960 with the blower mounted on the side that bellows is a push only design that has the trouble-some seal on the actuating rod. That is what failed on my car this year.
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'96 965, 16' wheels, Michelin Pilot Sports, rear 18mm bar + Koni, 204HP cams, 126K. Put 200K on '85 745 TD.