Ok ,, Not sure if this is goin to make since but here goes..

My 2000 dodge 5.9 gasser . In the summer months when trying to run the a/c it blows and cools fine. HOWever.. when you start to go up a hill the dash air stops blowing untill you crest the hill and or take your foot out of the pedal..
Im gonna burn up in the summer if I cant get this figured out.. ;;

I might add that some times ( hen climbing) I notice the air flow shift more to the defrost, hmm

 

Common problem, do a search for "wild vents"

 

+1, its a check valve

 

OK I pulled the one closest to the firewall ,, can blow one way ,, but seals tight the oposite direction. The other one I cant seem to get it off, dang.. So let that be theone?

 

Check for leaks downstream of the check valves as well. There is one line that runs off to a reservoir under the cowl. The plastic lines get brittle, and break with age.