Not likely to be trapped air. As Dennis and Joe stated, the pedal was not soft...it was not there.

I did have the same problem on my '70 'Cuda, a failed rear wheel cylinder caused the pedal to go to the floor while driving, and after replacing all four wheel cylinders and gravity bleeding gallons of fluid the pedal would not come back. I installed another known good master cylinder from my fleet and the pedal still would not come back. I took it to a repair shop that could not resolve it either, and they quoted me $1100 to replace the entire brake system which "should" solve the problem, but would not guarantee that they would fix it. So home it went. I never did resolve it, sidelined the car while I focused on caring for my son and growing my new business, and never did get it fixed.

When Joe had the same problem it stirred up some memories and a desire to help solve it by logic. Dennis and Joe together went through every other possibility besides a line with a pinhole leak (which would not show up in vacuum bleeding, but would show fluid with as much pumping as they did).

I just happened to obtain a spare '68 block the week before Joe's problem surfaced. I cleaned it up and examined it carefully and I now know what they do and how they do it. Yes, it's a "dumb" device, but things must be just so for it to do its job.

Hint #1: No pedal despite free flow of fluid.
Hint #2: Presumably 38 year old part, probably never been serviced or replaced. After all, they never fail, right?
Hint #3: No gravity bleed. Vacuum and pressure bleeding could move fluid through the circuit, but I am willing to venture that it was through a groove in the shuttle, not past the end.

I would be happy to do the post-mortem, and ask only for a detailed photo of the block as installed and the lines attached to it with a description of where they lead.