It has been mentioned on another forum that the alloys can be porous (??) and that a cure is to paint the inside with hammerite metal paint. Schrader advise changing the rubber washers on the pressure sensors at each tyre change look at http://www.schraderservice.com/howtofitme/
but I doubt if a tyre depot would have a supply of washers or would want to risk damaging a sensor as happened to me (which cost £50 to fix). I had to make constant visits to the garage to repressure the tyres and found it a complete bind.