[utopia]

Yes, I see your point, Nasher.
But are the float bowls not vented anyway (I'm not sure tbh), in which case there would be no air lock possible.
If they're not vented then how would fuel flow into them anyway ?
The only rationale I can come up with is that the float valves are sticking slightly, thus preventing fuel flow until the bowls have drained significantly, ie enough to crack open the float valves due to the added weight of the (no longer floating) valves ....?
I'm not convinced of this analysis tbh, but its all I can come up with.
..... other than "there is no problem", that is.

As suggested earlier, using low or zero ethanol fuel from Esso or BP would minimise any gumming up of the valve or deterioration of the rubber seat .... always a good ploy in my opinion, whether there's an existing problem or not.