I found the fucking leak. It was the glass viewport. The passivation ate the metal between the glass and the steel:
For the record: do not passivate glass viewports.
I found the leak by listening. Late last night the building was quiet enough to hear the leak. I didn’t hear a hissing so much as I heard the sound of the pump emanating from the surface of the glass.
NOW THE EXPERIMENT CAN PROCEED!!
I will let the pump run overnight to see how deep a vacuum we can get with just blanks.