I've got a 95 f150 5.0l vin N, flareside and when cruising on the highway or wherever no problems, but when i give'er the gas (just enough to pick up speed for small hills, or when i floor it) the air vents (with ac on, off, no matter what setting) always switch to defrost. Some air will blow from the floor and face vents but the majority goes to the defrost. I don't have a clue. I've checked the connector in the cab (4 or 5 different colored rubber tubes) and it's okay and checked vacuum lines under the hood (the ones i know of anyhow). Please help!

 

Under the hood where the vacuum line comes from the heater controls there should be a vacuum reservoir made from steel or plastic. Mine was steel and the bottom was rusted off of it. I replaced it with a plastic one and problem solved. Also between the reservoir and the engine or right on the reservoir where the line hooks to it from the engine there should be a check valve. Try sucking and blowing through this to be sure it only flows one way and that it will hold vacuum.