There are several methods for finding leaks and they all involve purchasing supplies and/or equipment. I like to start with UV dye. You can buy the UV dye for 134 systems at most auto supplies. It has an ounce or two of oil, UV dye and several ounces of UV dye, so you just add it like any other can of refrigerant.

Since you have a converted system, you must read the fine print and make sure it is for Ester oil systems. Once you charge the system, if the leak is REALLY bad you can see the dye with the naked eye, but it is best seen with yellow shooters glasses and a black light.

Since it's 134, I would go ahead and charge it up, putting in a can of UV dye first. You can buy a leak detection kit with a black light for not too much money, or maybe find some kind of black light to use.

Start looking for the leak at the compressor shaft seal, any joints and maybe the condensor. If you don't find anything there, then drop the passenger side underdash panel and check the expansion valve. If you don't see a leak there, get it cooling enough to drop condensation on the floor and shine the black light into the condensation water. If there is UV dye in the condensation water, then the leak is your evaporator.

Good luck,