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The most common sources of leaks are:

1. The test setup, and

2. The filler cap

3. The tank vent line

I assume you have double and triple checked all of those.

If I were in a quandary like that, I would think about submerging the
entire wing (pressurized) in a water tank of some kind and looking for
bubbles. Just buildja' a big box and line it with visqueen. Let's see, 60
gallons of displaced water weighs 500# or so, so that's how many sandbags
you would need to hold the wing down. Is the wing painted yet?

There are other more sophisticated options. Air Conditioning maintenance
companies have Halide leak detectors which could be used if you were to
pressurize the tank with Freon or Halon.

Hell, just fill it with 100LL and look for blue stains!