Maybe we got the answer further up, I’ve read all the replies but not all on the same day.

Regarding the bore size vs front brakes -
From my understanding, a small bore is for manual brakes, larger bores are for power brakes (to keep pedal effort within 'normal' for a human leg). There is probably also a slightly-larger bore suited for discs then, because I suspect that the fluid volume required to move 2 pistons is greater than to move 2 front wheel cyls. Also, the late-70s caliper bores are larger than the early 70s (at least as far as C-bodies go), which supports my hunch.

Using a small bore MC requires longer pedal travel/longer MC piston travel. There are also a few orifices/ports in there that I'm not fully versed with, but I do believe that if in the event of a pressure loss in teh front brakes (which run on the rearmost MC section) that the piston travels further forward and bump-stops the rear brake piston into action.

So climbing further out on the limb:
Small-bore MC takes lots of travel, can barely push the fluid vol to activate the calipers, but then physically pushes the piston for the rear brakes – and they lock up (especially because pedal effort is so low in this configuration).

Then one front brake is capped off, reducing the fluid vol requirements, so the pedal feels better, and blocking them both off makes a rock-hard pedal – proving the MC has integrity. The difference mentioned earlier, between blocking 1 caliper vs the other, is a mystery to me, though. Different length of metal tube was my initial thought but then discarded as the fluid vol required is same for both calipers and pressure loss in longer tube would be negligible.

And as a bit of evidence –
Almost 20 years ago I converted a 65 Fury from manual drums to power discs (c-bodies use a different pedal assembly for manual brakes, and you can’t tell the difference by looking down from the driver’s seat, you have to be aware of this difference). I used a 71 PDB booster /MC with my MB pedal. Braking was great but the pedal was very soft and felt like it had air in the lines. Bled them repeatedly, over several weeks, but to no avail. Blocked off the front hoses and got a rock-hard pedal, so I knew the MC was OK. None of the brake experts I spoke with had any good ideas. I didn’t drive the car very much, and the brakes were awesome, so I accepted it. I figured it out many years later as I learned more about C-bodies.

I think this is a related situation.