This is an older car, so I'm taking a guess here. What you are describing is either a vacuum check valve or a vacuum modulator valve. See if it has any lines running to the vacuum heater valve. It should have a least one coming from a vacuum source. Try and trace the lines. At any rate, whatever it is, it surely sounds broken. Get a shiny new penny and some new gum or try and remove it and get a part number and check whth the parts shop here, or your local, friendly, money grubbing MBZ dealer. Good Luck..

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE