Well...

My old bath plug to blank the exhaust was actually working fine!

Turns out air comes out past the power valve shaft if you haven't bothered to put the seal in yet.

I'm an idiot. That's what I get for trying to leak test with the bare minimum components on the engine.

Surprised it actually held pressure (mostly) without a powervalve seal.

5 PSI and holding!

Now.... how to test vacuum....

edit:

I got a tiny screw driver and held the valve while I sucked on it. Got it down to almost 5hg of vacuum - whatever that means!

Holding vacuum too. We're looking good.