Ok: About an A/C leak that is hard to find. (Jeez, the evaporator core?)

Please bear with me. I'm sorry if this is long.
2000 A4---1.8T---77K miles: Note, I am trying to fix an A/C system so I can sell my car. $1400 to replace the evaporator is just nuts at this point.
There is a leak in the A/C system that both my mechanic and dealership have concluded must be in the evaporator core because the dye just isn't coming out anywhere.
However my mecahnic is puzzled by something though: His company repairs Audi's a great deal and he said that since this car has had several recharges with 1 leak stopper that targets the O-rings one of which I replaced and at least 2 of the leak stoppers that is meant for heavier leaks with dye's both UV and bright red, that he is having a hard time imagining that even the evaporator core could hold that much dye + stop-leak oil base. There is no sign of a leak anywhere: clean condensor, weld points, compressor, lines, etc., etc.
Here's the whack job history:
1. June 2005: Mechanic 1 recharges system with dye. ---> a/c dead nearly right away 2. later June 2005: Dealership sees no dye and recharges with some form of stop-leak (I think) and dye. ---> this lasts through the summer. 3. April 2006: A/C dead---Dealership recharges---sees no dye----puts in more dye. ---> this lasts maybe a week 4. May 2006: Mechanic 2 recharges the system with a light stop-leak (not the "gold" type) and replaces an O-Ring and the Schrader valve. ---> a/c dead but took a few days to lose a small amount of coolant. The coolant is still there at a lesser amount but too low: the low-pressure switch has shut down the a/c.
6. June 2006: Mechanic 2 is encouraged by the slow leak and refills and drops in the dye and expensive stop-leak stuff. He lets it sit in his lot for a few days and it holds pressure and produces very cold air. ---> a/c dead in a few days. 7. July 2006: Mechanic drops in one more strong stop-leak and dye. ---> This is gone within a few hours. Can you tell me if there is any chance that the problem is not with the evaporator core? Any other thoughts?
THANKS!
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the minute something is charged to my account?
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I have put green neon dye in cars with leaky evaporators: You will see the green stuff mixed in with the drain water while running the A/C at max. I have replaced evaporators that had leaky o-rings, but that is the best way to do it. If you cant see the dye, then you need to confirm lhe leak by using an electronic beeper leak detector which can detect freon parts per million.
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Regal53 said something like:

I think he's done that, and cannot find a *thing* anywhere. THAT and it holds the vacuum for an hour, but then leaks out under pressure. And not always consistently. Once it took days. The last one was less than 4 hours (dunno how much less) before the low pressure switch cut it off.
Is it possible to have a line somewhere that is loose such that the vacuum pulls it tight, but internal pressure pushes it open??? Any other ideas?
Thanks!!!
PS. I'm sorry for bottom post formatting your reply----but I am trying to keep the conversation in order so that any interleaved posters would be able interject properly without creating a mess. Thanks.
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-Thufir Hawat, Mentat and Master of Assassins to House Atreides
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Highly unlikely. Try a diffrent technician/shop.
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Regal53 said something like:

Actually, the first attempt was the dealership. They gave me the stock answer: "Since we can't see the leak-dye anywhere, it must be in the evaporator core. $1400 please."
I'm really stuck as to what to do here. I'm trying to sell this car, and I just don't think that dumping another $1400 into it is something to do given my luck with cars: after the a-core is replaced someone will discover something else cheap and easy to get to that was the problem all along.
It's the way it works with me.

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