If the evaporator is leaking I don't think you are going to find the problem using UV dye, at least not without disassembling the dash so that you can see the evaporator.

It sounds like your A/C is not working too well. There are about 6 million posts in the archives about A/C problems, have you done any searching/reading?

Do you have any A/C service tools? Without a leak detector, pressure guages, evacuator pump you are not going to be able to fix whatever is wrong even if we magically diagnose the problem over the internet.