Had a leak at the back of the fuel cap, not only "getting wet" but really running down the tank. Dismounted the cap and found 3 sealings (O-rings). The 2 beneath the screws seemed OK but the one in the middle looked a bit strange. There is an O-ring but it is tight around the hard plastic part into which the small cone on the cap itself fits when closed. But the hard plastic part is 'deeper' than the thickness of the O-ring so it's hard to see any function. As the design seems to be for a wider O-ring I decided to try one and went for a test ride. The leak was gone but after a couple of miles the engine died. I opened the cap and it started again. Tried a couple of times more with the same result. Conlcusion: No or too little vent. Since I've totally dismounted the cap but I'm a little confused about the small channels, small vent holes and one-way valves. It seems that everything leads from the tank back to the tank with no chance to align any wacuum.
Thanks to all You great folks on this forum.