Hey guys, I have a 1999 1/2 Ton and when I drive around for a while, letting the engine get warm, and then stop at a red-light and let the motor idle, the flap that directs the air slams shut and directs all of the air to the defrosts. I checked the entire vacuum system and there are no leaks or anything. Anyone have any idea what else it could be?
Thanks

 

It's vacuum related whether or not you want to admit it. Bad check valve(s), pin hole in line, vac line under dash. control panel etc..