Did you look for dye in the condensation drain water while the system was still cooling? If there is dye in that, then the evaporator would be suspect.

For problem cases, the best way to find the leak is a shop that has nitrogen and a good electronic leak detector. They will put in 2 oz. of R22 and then pressurize the system to 200PSI with nitrogen. The reason for R22 is that it is much better detected by an electronic leak detector.

Hope this helps,