air conditioning issue

I have a 1999 dodge ram 1500 pickup . When I turn the Air conditioning on it works fine , when I accelerate on the highway the air blows through the top ( defrost area). When I release the pedal you can hear a damper close and it blows through the vents again. When I accelerate it blows through the defrost area again.

this is a verry common problem. the heat/AC controls are all actuated by vacume, check all the vac lines for leaks. there is also a TSB about replacign teh check valve in the line that goes from the engine to the heat controler under the dash, if you don't have any bac or leaking lines, you might want to look into that check valve.

Is this check valve under the hood where the line goes under the dash from the engine? it connects two rubber lines and has a flapper in it?

I can look when I get home tonight to make sure exactly what it looks like, but yes, it is under the hood, neer the fire wall, and is a little plastic disc.

Ya I think it's a straight piece of plastic with a round disc in the center of it