I had the same problem with mine. Be careful with anything that extends the original, I had some of those metal tyre pressure gauges on mine a few years back, the one's that show green, amber or red and the weight of it plus the constant screwing off and on must of weakened the rubber valve and it popped out at three figure speeds.
Went down rather quickly I can tell you..
Fitted these a few years ago, ordered in from Watling tyres for a fiver and been on the bike ever since. Being all metal bodied, no need to replace them every tyre change and so easy to check/inflate tyres.