1999 5.4 Triton. F150 2 wheel drive sport...
My a/c stared reverting to the defrost vent even tho the selector remained on Max air. It would only do this when accelerating then would return to normal. Now it always remains on the defrost vent / windshield no matter where I turn the selector.

I have read that it is a vacuum leak....true? If so where is a common / problematic location to check. I can hear a hissing like there is a leak when I open the Hood. I just am having a hard time locating it. Any advice would be appreciated! Thanks