Vic,
There is also a cork gasket under the filler cap that makes a complete and perfect seal with the filler neck of the fuel tank.
I have twice been stopped on my Comet the side of the road with an inexplicable die of the motor and no amount of coaxing would get a restart UNTIL I removed the tank bag - I could hear the whistle of the air rushing thru the hole in the fuel cap as it broached the unintended vacuum inside the fuel tank. Stuck a bit if roadside timber (small) next to the fuel cap to keep the breather hole clear - fixed. Never used a tank bag since then.
I have had a related experience with a car about 20 years back. Car rolled to a stop, engine dead and would not restart. After about 10 minutes it would restart only to die around 10 minutes later - what a circus performance - run, stop, run, stop, and so on. Turned out to be a baffle in the silencer that had come loose and blocked the outlet of the exhaust. When the motor stopped if I was quick opening the bonnet you could just hear the compresses gas in the exhaust forcing its way past the exhaust gasket manifold to freedom.
Martyn