Hello Silent Hunter,
the easy things are all checked, the rubber-pipes are all free, the pet-cock provides a good flow as long as the tank is full of petrol, the air-valves of the filler-caps are not blocked. As far as I understood the system, there could be only 2 more possibilities:
No. 1: The quick-detachable connector has collected dirt. I forgot to check it when I checked all the other components.
No. 2: There are some small diameter metal-pipes inside of the tank on both sides. You can see them leaving the tank were the rubber-pipes are connected. It seems to be a common failure that these metal-pipes get blocked by dirt. As they are of very small diameter it's not easy to push a wire through them, especially because of the double-curve at their entrance into the tank. I once tried and it seemed to work but only one or two times. Then it seemed that the dirt had found it's way back into the pipe.
Kind regards, Bambi
'Find me kindness, find me beauty, find me truth ...'
(Dreamtheater, Learning to live)