If you get the cans with stop leak which also contains R134a, they won't work. If you get the 3oz can to stop any leak, it will plug up your compressor if you ever loose all pressure, to say nothing about the machines the shops use to vacuum test AC systems.

Go to an auto parts store, like NAPA, and get a very small can of yellow dye made for AC. Then charge up the system. After a day or 2, use a paper towel and wipe down the receiver/dryer cannister and hope for a leak there.

Most cases, the leak is at the evaporator. Unfortunately, inspecting the evap through the small hole at the bottom of the dash is particularly hard to do. Some of the dye might show up at the drain hole at the center, bottom of the car, but even that is iffy.

From the sounds of it, you will need to replace the evap. A 10 hour billable job at the dealer, plus parts.

Klaus
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Common sense trumps a PhD

I'm no expert but some there some type of additive you use and can see with a black light. If you continue adding cans, I'd try using those stop leak type, but it probably won't solve your problem. Good Luck.