Thank you! My apologies for my original mis-information.

Openings on the right side (to EGR etc) capped. I've plugged every line on the left side of the engine, and specifically located and replaced each of the problematic vacuum lines and fittings.

I got to the point of isolating the components and found myself a bit stumped as I clearly don't understand how the vacuum amplifier or the vacuum component mounted near the injection pump (one with the red hoses going to it in the pic above) work.

The Mighty Vac will pull and hold a good vacuum from the vacuum amplifier to the transmission and the vacuum amplifier will hold vacuum when applied to the port in the center on the top.

So I'm pretty sure the point I'm sitting at now is that I have the transmission portion of the vacuum system isolated (hopefully), have tested all of the associated lines independently, and have tested the top side of the vacuum amplifier.

Any ideas on how to proceed?