Whenever i have a load on the engine my vents seem to change. it will go from the vent to the defrost. I think i am loosing vacuum. i had the engine compartment apart a little while back. did i forget to install a one way valve some where? or do i have a problem with the diaphrams in the heater assembly? Can someone let me know. really annoying problem.
thanks

 

What year is the truck?

2wd or 4wd?

This check valve would be the first thing to check/replace: http://dodgeram.info/tsb/1996/24-01-96a.htm

 

I have a 1995 4x4

that seems to do it.
thanks