{Did you verify the fuel vents are working properly?   ....and that the check valves on the vents are oriented correctly?}

There should be no check valves on vent lines. If you take off at sea level with a wing tank at atmospheric pressure, 14.7 psia trapped by a check valve, and climb to 18,000 feet, static pressure 7.35 psia, the differential pressure on the tank will be 7.35 psi, 1,058 pounds per square foot trying to rip the skins off the spar and ribs.

Vent lines must be free flowing both directions.