Recirc problem in a W140

[pcmaher]

I did some searching on the forum, but could not find anything specific to the problem I'm having with my 1995 S600 W140. Problem first noticed today. Hot day, A/C on, turned recirc on to cool off faster. I could hear flaps moving, and air flow sound distinctly changed as it should when turning on recirc. As soon as I pulled away from stop, heard air flow sound change, heard a slight wheeze / whistle with pitch change, center vents stopped blowing, and air flow started coming out of defroster vents. As soon as accelaration was stopped, recirc functioned normally again...slight wheeze / whistle with pitch change, defrost vents shut off and center vents started blowing again. It definitely is related to engine vacuum levels, and only when in recirc mode. No oddities when in normal mode. Low vacuum levels have something to do with the problem. Coincidentally, my 2-3 shift seems harder than normal. I could use MBDoc, Benzmac, similar guru help. I believe there is a vacuum reservoir canister in this car. A diagram I found shows lines for the climate control and the transmission being fed by this canister. Could this be a potential problem? What else should I look for?