A vacuum leak anywhere in the system will cause that. And yes there is a check valve as well. Even the slightest vacuum leak in the engine bay can cause this problem. What is happening is that when accelerating the vacuum drops to low for the HVAC system to keep the proper pods open and closed. There is a check valve and a vacuum resevoir on the HVAC side of it to keep vacuum steady during acceleration. But if a pod leaks or there is a cracked rubber connector on a vacuum line they you will have a loss of vacuum causing the HVAC to default to its no vacuum state which is full heat, opens the blend air doors, and also full defrost since that defaults to open as well. So you could have a bad pod, bad rubber connector in the vacuum lines, bad check valve, bad resevoir or line to it, or a leak in the engine bay.

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....