F150 Air Conditioner Problem

I have an odd problem with the flow of the air conditioner. I had to have the front brake lines replaced because of corrosion, 77500K, in fact one actually perforated. Right after that the problem with the air flow began, maybe just a coincidence. The air flows from the panel just fine until I begin to accelerate, then the air is slowly deflected to the defroster and stays that way until I let off the gas. At that point flow returns to the panel. I can hear the doors changing. I checked the vacuum line from the vacuum booster to the junction at the intake manifold and found nothing loose and no evidence of rodent activity. I see nothing up under the dash board that looks loose or chewed. What are some possibilities? I guess it goes to the shop soon.