I agree with the comments that the mechanic you have now is creating more problems than solving them. A mechanic who fills a/c system with air does not know what he/she is doing. Change the mechanic right away. If the leak was through the compressor, a proper mechanic should be able to tell you immediately, there are refrigerant leak detectors that can easily find this. The leak that is real difficult to find is in the evaporator behind the dashboard, but this too is generally inferred based on not finding a leak anywhere under the hood. Also if you keep filling R12, at some point you also have to fill some oil, with each leakage episode, you are also loosing some compressor oil. Use a heavier oil, i.e. use StopLeak or something like this to fill some oil along with freon.

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Saumil S. Patel