My pedal works fine with my master plumbed either way but it's also a disc/disc p30 master.
Does it matter if you have the vacuum hooked up or not? I'm assuming you were bleeding with engine off and the started it to check how it was.
What if you get it to stick to the floor and then unbolt the MC from the booster and see if the MC piston is stuck inside or if the booster/pedal assembly is giving you issues. Sounds like fluid is just not returning correctly.
Has it been having the issue with only this MC?
So weird that its done it with multiple MCs and boosters. There has got to be a common issue but then the trail ride in the middle with no issues is another weird thing. I have never messed with the vacuum or check valve. Always had everything stock. Is there any thing to go on there since you said it comes back with vacuum removed or truck shut off.

Oh, I'm sorry, your inflicted by small cam syndrome. Must be hard to face the wife.
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Curious if when the pedal sticks, that opening a bleeder or line somewhere in the system may/may not cause the pedal to return.
I was to understand the theory that the pedal is returning only when the fluid is returning.
That is barring any issues deeming multiple booster failures