Water-pump's back together again! Surprisingly, it didn't gush water out of it as fast as I poured it in. In fact, it seems to be holding water very well, a couple of loose hose-clamps aside.
(Already had some slight complaints about my burble being somewhat incomprehensible, I'll add captioning at some point. Probably after work tomorrow.)
So, I decided to trundle it on over to the machine-shop that re-made the flange I broke.
Getting it back over and parked up was an exercise in frustration; which (un)fortunately, I didn't manage to capture because I forgot to turn the camera back on. It took me about as long to shuffle the truck sideways in shallow S turns, as it did to drive to the machine-shop. Combined with a very VERY warm cab -- no engine cover fitted, it's next to my work-bench, awaiting rust-treatment -- it was a trying experience.
Not prodded and poked at the brake servo since my last post, since I was out all 'weekend' (the Weds/Thurs); I'll have another look at it and see if I can figure out what bits I'm missing.
I think I'm actually missing the plunger of the assist valve completely, though; so that'll be interesting to deal with.
One of my next steps may well be to get the fuel tank cleaned and painted, ready to go back on. Not convinced that the level sender is working as it should; doesn't appear to be a smooth resistance curve from full to empty... jumps around a lot, going open-circuit at a couple of spots up the rheostat. May just be corrosion/deposits that might clean off.
Long shot: Does anyone have a rough idea of what resistance I should be seeing between the sender output, and its earth, at the limits of its travel? (The WSM doesn't seem to list it, unless I've missed it.) If not, I'll just have to figure out what it should be, by the "lots of error" method.