I haven't found my particular problem after seaching the forum. The "flapper door" on my '91 300SE wont stay on vent with the A/C running. There is plenty of cold air but it comes out of the defrost instead of the dash. I can "play" with it and get the air to come out the vents but as soon as I have to push down on the gas it perhaps is changing the vaccuum and out the drfrost it comes again. Any ideas of what could be the problem, from what I have read I don't think the monovale is at fault because I can control hot and cold.
Thanks Everyone

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1991 300SE (my ride, 279,000 miles, looks almost new
1954 Cadillac (21 yo son's car, he bought when age 15)
1972 SeaBird 19 ft runabout (old but solid, slant six, Volvo sterndrive perfect condition, undergoing complete overhaul and refit)
1998 Toyota Rav4 (my sons daily driver when he is in the Continental US, PROUDLY serving in US Navy)