I believe there is a seal and valve "assembly" under the threaded end of the cylinder. It might just need removing and cleaning/replacement "o" ring etc.
One of my 2001 cylinders could not be dismantled and had to be replaced.
It is worth ringing Bob at Checkmate Guns as he serviced my SAM for me last year and it has been fine since then.
http://www.checkmateguns.co.uk/
Hope this helps.
ATB
Mike.