Since your leak seems to be a very slow one, leak detection can be difficult. The normal process is to inject a UV dye into the system and use special goggles to view possible sources. But as I said, the odds are very high that it's your evaporator.

At this time, I would simply recommend that you have the system evacuated and recharged in the normal way without any *sealers*. If you can go another four years, or even two years, or even one year, it's a lot cheaper than fixing the evaporator. If you're a DIY'er, it's possible to charge the system yourself fairly easily (<$15), though you won't be able to do it as accurately/optimally as a shop with a machine.

Gerald C