I tested my Legacy wings by plugging all but two holes and covered the vent and fuel cap (latched) with packing tape.  I then installed barbed fittings to the two remaining threaded holes and put a party balloon over one and a tube on the other.  I then compressed the tank and balloon through the tube until the balloon was filled and capped the tube.  I checked all the plumbing and capped holes with soap bubbles, but saw no signs of leaks.  However, the balloon was partially deflated the following day.  A tied-off test balloon didn't deflate measurably, so I must presume that the air didn't bleed through the balloon.

Does anyone have any ideas for further leak tests? 

Kyrilian,

 

Yes.  Did you then reverse the balloons?  There is no guarantee that the one on the plane did not have a leak (presumption).  Otherwise, use high quality condoms as the Surgeon General's Office warranties that they are free of leaks.

 

The other problem with these tests is a change in the barometric pressure.  Higher baro, smaller balloon and vice versa. Maybe a better test would be to use a water tube loop mounted vertically and calibrated in inches.  Take a barometric reading when sealed and also the next day and correct the resultant water level reading.  This way use can test with both positive and negative pressure.