zini are you sure there isn't just a bit of debri stopping the valve closing,maybe pushed in when using the fill probe?
this has happend a couple of times to me on differant FX rifles.
it maybe because the FX guns don't come with a protective bung to fill the hole for the probe when not filling.
its an easy fix,just quick strip and clean but of course the rifle must be EMPTY OF AIR before starting.