Hi Alfie
The one way valve is supposed to let air in. But you also need to let air out or the tank will blow up like a balloon when it gets hot (weather). I took the valve apart and made a little cut in the seal. It now lets air in (quickly) - but also lets air out (very slowly). If you don't do this the expanding fuel and air (in hot weather) can actually force a fuel leak.
If I am ever unfortunate to make the car go upside-down I can relax in the belief that fuel will only escape very slowly....
Robin
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