Any help would be appreciated. 1996 f150 5.0 , the a/c works fine except it stops blowing through vents when going up a hill, and starts blowing again when I get to top of hill. Now recently the defroster and floor position blows cool air. Coolant is full, 2 lines into firewall hot. Thanks for any help. Winter is coming, and I am old and skinny.

 

You probably have a vacuum leak somewhere.