Ok. Well done.. Float vent would be an obvious one I guess.. in hindsight !! It's a concern to have to admit to having seen that before.. Dang... Memory and focus isn't what it used to be. And I was sitting here with that feeling that the answer was right on the tip of my tongue.. That I'd seen that exact symptom before..haha. And yeah, I guessed you must be some sort of mech to be using a vacuum gauge.. That's old school.. and a bit of a lost art. Never hurts to brainstorm though huh.. sometimes just talking it through helps our thought processes ( I wish).

As Ulfthendar says, it would be a good idea to deglaze the drums.. rough emery tape or sandpaper.. 80 grit, and a bit of pressure.

Oh Ok. The barrel type adjusters are easy to adjust, you just tighten them till the wheel won't turn, then back them off till it does turn without drag. A light scuffing of the shoes is ok.. It's important you do them till it won't turn first though. If you just wind them out till the shoes touch, the shoes can be touching at only one point and will get more clearance once they centralise. Locking them right up ensures they are centralised so they don't reposition themselves when you use the brakes. I'd try locking all four shoes right up and then bleeding the brakes. That way you know there's no lost travel in the hydraulic fluid having to push the shoes out as you are bleeding. You should be able to get a good hard pedal with very little travel doing that.. If you can't, then you might need to try bleeding a different way. I generally pump the lever fast till it won't pump up any more, then still holding it down, release the bleeder as far as possible as fast as possible. If you pump the fluid through slowly, or release the bleeder too slowly, bubbles of air can rise up the pipe and not get flushed through. If there's no travel in the shoes, you know whatever free play or pumping up there is happening is in the hydraulics. The other test of the bleeding is, lock the shoes with the adjusters, pump the lever till it's hard, take note of how far it is from the handlebar, then let the lever off, wait about five seconds, then pull the lever in once and see how far it is from the handlebar.. It should be the same distance as when it had been pumped up.. If you can pump it up it still has air in it.