No one really knows what is causing this problem. I believe the aluminum evaporator is being eaten from the inside. There is a theory that the material from the brushes in the blower motor cause it. There is a leak stopper product that is very good. I would not use the walmart junk (Interdynamics, a bunch in New York that are not very knowlegeable or helpful). The company is Cryo-Chem in Brunswick, GA. It is a two part kit and they want you to replace the dryer first. The kit is approximately $100. First, the drying agent that removes all moisture from the system (this also gives an incredible performance increase) and then the sealer. This product will not harm your system and will plug up the hole in the evap as long as the size of the hole is within the specified parameters (as with all sealers). If you don't do it yourself, you will have to negotiate with your mechanic.
I replaced the evap myself with an aftermarket unit because I have no faith in the factory evap. Again, if you remove all the moisture from the system(hence most all the acid) then a new unit should last forever.
Most good sealers are moisture reactive(that is how they seal the hole) and I believe the only way to insure that they work without presenting a potential problem internally is to replace the dryer first and insure that the system is moisture free.
Hey it's a crappy problem, but most of us have been there. Good luck

Lots of stuff on web site about this common problem of leaking evaporators. The option I used, like many is to go to Walmart and get recharge kit and sealant and learn to recharge yourself. Initial kit is about $35 and additional R34 refigerant is $10 or so. You can this many times before spending $1200 to $1400. I found found I now need to charge only once or twice a summer. Look for more details on this site.

This is typical. I don't know of many who have NOT had this work done.

Sorry and good luck

The only less expensive idea would be to do without A/C alltogether. Assuming your mechanic was correct in diagnosing the problem, and that your A/C failed a month after recharge, indicates that the leak in your system is not small enough for any of the sealants to take effect. Replacing your core is the only solution.
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'82 240DL Stock / Masters Class, '94 850 Fully Loaded / Daily Driver, CVC / VCOA Member http://www.capcan.com/tonyg.htm