Air Conditioner Problems

A/C system leaks can be difficult to locate, especially when they take months to leak out. What a good shop will do is add a fluorescent dye to the refrigerant, then after a period of time, they will use a black light to find traces of the dye, and this will pinpoint the leak. The amount of time depends on the size of the leak, and in your case I would say it would be visible after a couple weeks. Now there is one problem, evaporators are buried inside the dashboard, and often times you cannot visually see it without at least partially removing it. So if there are no visible leaks anywhere, then the evaporator would definitely by a culprit.