I couldn't screw the Gunson adapter cap on tight enough to get a good seal, their neoprene one with the kit was simply too hard.
So as soon as I applied air, the air quickly expelled from the reservoir and fluid went all over the place, fortunately it was silicon brake fluid, so just a mess.
I replaced it with a home made cork and neoprene seal and then things went well.

I also have a theory why vacuum bleeding doesn't work and it has to do with the external brake booster and how its air valve is designed as well as the piston vacuum chamber.
If you look at seal 15 in the diagram, if one applies vacuum to the system, it could (and probably does) pull air past that seal, and quite probably past seal 46, but not so much with that one because there is a spring pushing from behind on it.
Seal 15 is quite weak and is designed to keep the pressure of the fluid in, not to have suction applied to it from its back side.
No matter how much I bleed with the vacuum bleeder, the air would not stop, I even put heavy wheel bearing grease on the nipples to keep air from being sucked around the threads.
It may work if one only apples a few pounds of vacuum, but I think anything beyond that, and air would be sucked pass said seals.

 

Hi Jeff,
Glad you had success with the Eezibleeder.
Re the seals in the Servo, air should not pass either of those seals unless there is a problem, if you imagine that the fluid pressure behind the seal under braking would be somewhere between 800 and 2000 psi then fluid would pass the seals to the outside if it was possible for air to pass the opposite way. You would have to apply a lot of vacuum to deform the seals enough to suck air in.

Seal 46 due to it's shape would in theory seal better the more brake pressure is applied, of the 2 seals that would be the only one that I think may possibly deform under vacuum to allow air past, but when vacuum bleeding you should be using about 30 in Hg (1 Atm) and this should not cause any issue. It is very common with vacuum bleeders to get constant air bubbles from air passing the nipples or the attachment, pressure bleeders are the only way to go in my opinion.

 

All the bleed nipples had heavy wheel bearing grease on them, so no air was getting past the threads.
It would appear to bleed OK and the bubbles would begin to be less and less, even the peddle would start to feel solid.
But then I would move onto another caliper and more bubbles would come, then the first spot would have bubbles again _ it just wouldn't stop.
Came across a write up with a guy and an XK, same thing, bubbles just wouldn't stop and he couldn't get a hard peddle.

I would start with the caliper furthest from the reservoir and follow procedure, but it didn't make a difference.

It doesn't matter anyway, the Gunson bleeder appeared to have done its job.
I'll know for sure when I take it for a drive _ still waiting on a few parts though.

 

At the point you were at I may make the same conclusion, sure is an odd one, but as you say, the pressure bleeder worked, so long as the brakes are good all is well.

 

I don't think vacuum bleeding is very efficient, I used that method on the clutch system, and it too, took forever _ bubbles never seemed to stop, but I eventually got it.
With that said, if it took that much effort on a small system, then I could see it taking days, or even longer on a large one.