I had a leak in my system, but it would take about 2 months before all the freon leaked out completely. I've had dye put in it several times, but the leak was never visible so it must be in evaporator core. This last time I had it evacuated and filled up I put a can of freon with stop leak in it & I'm hoping that this might help since it's such a small leak. I was just trying to avoid having to take the dash out. I've heard good and bad things about using the stop leak though.

 

I replaced my A/C and heater core 2 years ago. I also had the disappearing refrigerant mystery. Die in the freon - no visible leaks had to be evaporator. Wrong! I still had the freon leaking.
I finally got my own cheap freon refilling kit because I got tired of paying shop rates for a simple A/C recharge.
This spring I put in some leak stop. So far the A/C still cools. I resisted using stop leak for fear of creating more problems. I read on the forums where someone else had tried this with no problems.
I would have to recharge the system every 2 months and it is 3 months now and I am still okay.
I replaced the heater and evaporator myself and would not hesitate to do it again. I am not fast at these things but considered that a good education that I can do this. Just take your time and follow the steps provided here. There are hints here which are not provided in the shop manual.
The one thing which almost stopped me was the little tool which is needed to separate the A/C lines. When I glanced over this thread I did not see if that tool was mentioned.
If you replace the evaporator also do the heater core. My heater was not putting out full heat as it was partially plugged. I was amazed at how deteriorated the fins on the evaporator were from junk which had built up it.
Stan

 

so, does this mean that the evaporator is the source of a leak when you have yellow dye mixed in with the water coming out of the drain hole?