I set it up this way right off the bat. Does your arb inner line have a seal on it? Mine has a compression fitting inside the housing with a brass ferrule.
It's coming back to me. The air line goes through the bulk head fitting to a little copper tube that goes down to a disk that fits on the center section. I have that part, the disk and tube, as a spare, I can look at that. IIRC it is a compression fitting.
Thinking again, I may have used 1/8" tubing for the diff locker. If so, I bet just a 1/8" ferrule in your existing setup would seal adequately.
5mm = 0.19685 in. Now I have to go measure..
Edit, I went and looked.
The rear diff is set up in a similar fashion to the pic I posted, but no brass tube support. When I set mine up it was a used unit. However I drilled and tapped the diff for it. I do not remember if it was engrish or metric pipe threads that I tapped.
I took the bulkhead fitting down to the local hose store, they grabbed a piece of 1/8" air line and a brass ferrule for it and put everything together.
I suspect that if you pull the center compression nut out of your diff, and take it to the local hose store, they can set you up with whatever you need.
Oriely screwed up and lied to me to top it. They won't have the parts until Tuesday. Oddly the dealership can have them by tomorrow, they usually quote months on anything related to my FJ40.
Changing the lines out looks like something that will have to wait until I revamp the whole air system. I'm going to see if ARB USA is open tomorrow and order 20' of new 5mm line.
I found this to uses as a surge tank
This is a little bothersome
Since I don't plan to hold more pressure than it will take to engage the lockers I'm not all that concerned.