My AC stops blowing when going up hills. I did a test where you run the engine and max ac for 30 sec then shut the engine off while keeping the max ac blowing. The max ac automatically switched to defroster after a couple seconds. I did the test for regular ac and it worked fine and did not default to defroster.

So I inspected all the vacuum hoses under the hood and all were fine. The black and gray hoses that enter the cab is where the problem starts I think. I blew and sucked in both and air went both ways in both. So I went in the cab and disconnected them where they come though the boot. I plugged them with a nail and blew in them and they don't leak.

I am clueless from there on. Could it be the vacuum check valve? If so where is that located. Any help from you guys would be appreciated.

 

AC stops blowing when going uphill?

So the blower fan stops not the AC?

Or the blower fan stays on but AC gets warm?

Or the blower fan stays on, AC stays cold but Blend Door closes stopping air flow thru vents?

AC compressor is suppose to shut off when gas pedal is at full throttle, can't change that one.

Since you think it is a vacuum issue check the vacuum reservoir(VRES on diagrams), it usually looks like a ball with vacuum hoses attached, this "ball" holds a vacuum for use when engine is under load and vacuum in intake is low, if it isn't holding a vacuum then vacuum operated devices would quit when "going uphill"

 

The air stops blowing out the normal ac vents. I can still hear the air blowing but it sounds like it moves to defroster. I took it out yesterday and found that it only does it on max ac not regular ac.
Why would the max ac lose vacuum when the engine is shut off and not regular ac?

 

Found this at another site, pretty close to your issue: Air defaulting to defrost, Blend door or vacuum leak? - Ford Truck Enthusiasts Forums

 

inspect the vacuum ball on the passanger side inner fender. may have a crack in the ball or a cracked line going to it. it stores vacuum for the times the engine isnt producing vacuum.