Gas Cap is leaking

Here's the story:
I got a new steel tank to replace the original fiberglass roadster tank.
Its a Pakistani tank. Seemed pretty good.
I had it pressure tested. It ran 6 Lbs for 3 hours with no loss of pressure.
I'm sorting out the rest of the bike and using it.
When I lean the bike over as a test the cap, it leaks gas.
I disassembled the cap and it seems ok.
I stretched the spring a bit to provide a little more downward pressure as to seal the rubber to the rim.
Still leaks.
Now I have a new one waiting in the wings for after the paint job once shes fully sorted.
If that leaks I'm really gonna be bummed.
Any tips or tricks?
I've cut a circular piece of wetsuit material and use it as an additional gasket.
That seems to work.
I will replace that with a piece of real rubber from an innertube.
But theres got to be a better solution?
Thanks in advance.
JA

There is a small vent hole thru the metal disk that holds the rubber cap seal in place.
This is to allow air into the tank as the gas goes out.
If fully sealed you would get fuel starvation.
I don't think the cap is intended to be totally liquid tight and unless the bike is really laid over the gas really shouldn't ever get to it.

Bob

Is the cap an old one if so the seal is probably shot. It grows with age and the sealing lip misses the seal face on the filler neck rim. A new seal is less than £2 I'd just fit one. Bob is right, the cap can never give a 100% seal.

Cash