That's that bit complete! (Will need to put a little protective sleeving on the plastic pipes where they pass right by the braided hose, just to remove any risk of rubbing/melting.)
So, we all know what needs to happen now, don't we?
Of course, my little temporary fuel can ran out of fuel before I got to the point that the regulator hit the cut-out, so I didn't get to completely check how that works, but it all seems pretty promising so far! There's still a couple of little leaks here and there that I need to sort out, but this is a major moment for me.
Both gauges reading pressure, warning light extinguished (and if the buzzer was hooked up, it would've stopped sounding too!) I still need to time how long it takes to build air up to an effective minimum level as per the inspection manual's assumed 3 bar/45 psi, which should "normally be within 3 minutes".
Interestingly, I did notice that the "Rear" tank (for the trailer) didn't seem to start building pressure until the "Front" (truck) tank reached 7kg/cm² (6.8 bar), which really drags things out as far as reaching that minimum level on both tanks and turning the warning off, especially with the larger air tanks. Might have to adjust the 4-way valve elements to even it out a little.
Still have a few things that need finishing up, like mounting the trailer park valve, quick-release valve, and properly mounting both of the trailer gladhands. I did fit the yellow line glad-hand in place of the old one, just so the trailer brake line isn't just a giant air-leak, but the newer ones are shaped differently and I'll need to change the brackets to give a little more room to actually hook them up.
And it'd be nice to get the fuel tank put back on its brackets, because it's extremely in the way at the moment, and I want to get back to welding the inlet manifold up. It'd be a real bonus if I did the rest of the fuel system stuff to be able to use the in-tank pump, too. (Regulator, filter, etc.)
On a more annoying note, the gearbox has started leaking from the PTO
flange cover again. Not sure why, because it's been fine for months,
but there it is. 
Going to have to drain the gearbox down again, and
apply a nice thick gasket with a good coating of sealant.
The following day
Brought some more fuel, gave it another go. Got all the way to reaching the main system pressure (about 9 bar) and it blew-off as expected! Still haven't adjusted the protection valves yet, though. A little under 3 minutes to reach ~3 bar on the 'front' tank, but then waiting for it to reach ~7 bar before it starts filling the 'rear' tank leads to it being a good bit over 4 minutes before it reaches the same 3-ish bar threshold there too.
I also managed to get around to modifying the bracket for the footbrake valve so now it's inline with the relay arm, and added some bracing; still needs an additional mounting tab to reach up to a bolt on the master cylinder for some support, because now it's flexing at where it's bolted to the back of the servo.
In the process, I did manage to drill holes for -- and rivet on -- the brake light switch, only to find out that where I'd fitted it, it runs straight into the underside of the master cylinder when I reinstall the bracket. Hm. Some additional thinking needed there, then. Also appears I need to lengthen the pushrod from the relay arm to the footbrake valve, because it's just a bit too short to actuate it beyond just barely cracking the valve open. Not a big deal, though.
It's all just adjustments and tweaks!