2 minutes sounds pretty quick, if not an o-ring and there is no obvious leak, did you also go up under the dash above the gas pedal at the expansion valve? Also, the compressor front seal could be leaking, in which case you need to hold the leak tester just below the compressor because the freon ofter disspates beofre the meter can pick it up.
Personal experience: I seldom use my leak detector any more. First I look for the ref. oil on the entire system. If there is a major leak, the oil is there also. I always charge the AC system with dye, that way the small slow leaks are seen with a UV lamp.