97 Expy HVAC issues

[rallye72]

I'm assuming I have a vacuum leak on my heater box because when driving I hit the gas my A/C willl go from the vents to the defroster. Is this a common problem? Whats the normal part that goes wrong? Thank you

 

[ELVATO]

Not completely sure, but I think that is the problem when some of the vacuum hoses by the windshield washer tank crack open. I'd have to search.