leaky gas cap

[1934]

I'm getting the '34 ready for inspection and I have a problem with the gas cap. Doesn't matter how much gas is in the tank there's always a leak from the cap. I made a seal from an old inner tube but that didn't help, then I made one from cork and it's just as bad. I thought maybe the vent was blocked, (the one on top of the tank), so I blew air through it and it seems ok.
Should I just buy a repro cap or is there a way to make a proper seal on the old one?
Thanks, Doug.