Hey Chris/Lew ?(Your name keeps changing),
I explained about the master cylinder before.
Did you read the part about master cylinders introducing air?

It's not a big deal to pull the M/C
It's only 4 brake lines and two nuts on the firewall, and a connector
and its not buried under the intake manifold.
I wrapped the open brake lines with aluminum foil to keep dust out\while bench bleeding.

Read the part about two small brake lines cut in half to hook up to the M/C
and clear tubing dead ending in the reservoir then you can see whats happening.
A boneyard M/C would be a crap shoot, BUT you could score the fittings (Old
brake lines) that you need for bench bleeding
The fittings supplied to bench bleed my rebuilt M/C were junk
and a giant PIA not to be repeated.

The rebuilt M/Cs were also of poor quality I think since it looks like the deep part of the M/C bore must suffer corrosion and honing wont clean it up right.
Next time I'll get a new M/C.

Bill