So this morning I took the pump arms out of my 107 and 108.
The 108 arm was about half a cm shorter.
I adjusted the 107 arm to be exactly the same length.
But the 107 forearm still drops down when there's no air in the valve
And the forearm opens up again when I pump the cocked rifle and then open the arm up a little. So there's still a bubble of air in front of the pump cup, which should have gone into the valve.
Perhaps the 107 has a different position of the valve wall in de tube?
As in, further towards the rear of the rifle, so that I actually need to make the pump arm quite a bit longer than the 108 one?
From the outside the actions look identical.
Hmmm.
Will have to do more testing - hopefully tomorrow