Air conditioning not working

Hi,

My car had been having issues with A/C not blowing cool air. The fan works but will only blow hot air. I've taken it to three different mechanics, they've done the dye test and a full diagnostic on the A/C. None of them can find anything wrong nor can they find a leak. They put more Freon in the car and then the A/C works for a couple of days then it goes back to blowing hot air. They believe there might be a leak where the dashboard is, as it's the one place they can't look without taking the dash apart. That will cost more then I'd like just for them to take the dash apart. Could it be like they said, a leak in the dashboard? At this point I'm not sure what to do.