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I just did a very rough ballpark calculations and based on that in the best case scenario (carbs are level the the bowl is full) to overcome the pressure of the float on the valves one needs about 28psi of pressure (2kgf/cm2) . Gasoline in when it is not blocked by petcock only creates around 0.56psi which is way too low to open the valve.
And if you think about it even for bikes with a petcock opened by the vacuum the petcock state is equivalent to open state of a regular petcock yet the mixture is not affected bu the petcock. If valves could not hold the gasoline back then the mixture would be affected by petcock that is it will be modulated. The card would not work properly ever. It would be impossible to tune.
You might need to inspect the valves - they might have imprints on the rubber which will not align to the edge of the valve and will cause the leak. Worse yet the orifice could be damaged or there could simply be some debris.
I do not think you can detect this kind of conditions with blowing the air. The leak could be very minor to be detectable by air blow. Yet over many hours it will let enough liquid through to be noticeable
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