Line 2 valve leaking

Hello everyone.

On my Vintage Line 2 the valve were you fill the gas is leaking. After filling you can hear the gas going put until its empty. Any tips or easy fix? Or do i have to order new O-rings and the speacial tool to fix it. If so, how big is the change to fix it that way or is it common that it is not the o-ring which is broken and i need to exchange something else.

Also is sending it to Dupont an option or even possible?

Thanks a lot!