>>1745117The fuel mostly leaks through the wick (this is it's main function after all). There's also a little leakage between the gap of the case and the insert, but that's minimal in my opinion, though some people say that major leakage comes from between the gap.
Problem is: the case's cap is not airproof, so fuel will leak out to the outside world. If it was completely airproof, then evaporation would stop relatively quickly after filling cap chamber.
There's also this gasket thing:
https://www.youtube.com/watch?v=DfLx2PTiK24 