A couple things can cause that big of a leak. First the gasket may be damaged or aged and no longer seals well.
That is a somewhat common issue after sometime in service. Years ago a solution was discovered that takes care of this issue. Rubber gaskets are used with the stock caps. They lose elasticity over time and don’t seal well. I’ll attach photo of a permanent solution. It is a blue gasket available through Mercedes dealers. The photo has the part number on the package. This gasket is somewhat thicker than the stock gasket and you may need to adjust the tabs on the caps due to the thicker blue gaskets. This blue gasket does not seem to age and lose shape or elasticity or be affected by fuels. These gaskets have solved many cap leaks over the last 18 years or so. I installed a set in my first Kitfox 16 years ago and they are still in service with no leaks. I hope they are still available from Mercedes under part number 140-471-00-79-493
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Secondly you can J.B. Weld shut the underside of the crimped rim of the cap and the four tiny “vent” holes found on the rim of the cap. Not the ram pressure tube hole in the center of the cap that is there to help pressurize the fuel system in flight. Either one or both of these fixs should end the fuel leaking during flight. Likely the first fix will take care of the problem. Hope this helps. I do these modifications to my caps and have never had cap leaks.