if you cant get the valve to seal attach one of these and leave it on, they come in straight versions too, or pack a car tyre dust cap with a little PTFE and tighten on.
http://www.ebay.co.uk/itm/45-Degree...pt=UK_Motorcycle_Parts_13&hash=item41734a2315
http://www.ebay.co.uk/itm/45-Degree...pt=UK_Motorcycle_Parts_13&hash=item41734a2315
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