There's no doubt that it's finally venting through the two oppoaing ports in the filler valve.
I even caught it leaking through the hole in the forward end of the screw-on valve cover.
I think there's a very slow leak somewhere that is too slow to be heard or felt.
The dogleg fitting is i the obvious questionable area because i had ro reuse the old screw, which
had lost its hex hole for an Allen key - had rounded off. So this screw currently has
a homemade screw slot so it probably isn't tight enough. I'm also certain that the seal at the tip
of the filler valve 'bullet' is being made by metal to metal contact...the o-ring is definitely not
touching down the the chamfer.
At present, the gun is with my mate and he's on holiday so it must wait. Meanwhile, I'll get new dogleg o-ring and a new fixing screw and try some leak testing at 190bar.