| I posted an answer to this a couple of days ago but it seems to have got lost in the server changeover.
Anyway, the valve should have a little arrow on it pointing away from the tank. It should be possible to blow easily in to the tank direction but there should be a slight pressure required before it will blow in the outward direction. I think this is to prevent fuel surging out under braking. If you can't blow through it in any direction, it's buggered. | 
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