"Here, you are only considering three possible bore sizes, 7/8", 15/16" and 1" (and their metric equivalents)."
Actually, if you take another look, I considered the four bore sizes mentioned within the posts and figured the ratios for each.
"7/8" bore will give you more pressure and tend to lock up the brakes. 1" will reduce the tendency of brake lock up."
And will yield a much higher pedal effort, even higher if you used the 1 1/8" master. That's the ratio thing. If the larger masters were acting against proportionally larger larger slave bore areas, I believe the pedal effort would remain the same.
"You do not reduce the presure to the rear brakes by using a master with two different bores. You use the same bore for front and back and reduce the pressure to the rear with an inline pressure reducing valve, i.e., proportioning valve."
Except Pantera proportioning valves do it by reducing pressure to the front.
A stepped master will invariably reduce the rear master bore, as it is the deep inner piston. The closing of the masters internal feed ports as the master piston advances under pedal pressure will result in inadequate volume to the rear circuit necessary to take up the slack and build real pressure at the rear pad/rotor interface.
"Brake line sizes also come into play here. The stock Pantera lines are equivelent to a JIC -3 size. If you enlarge the system to a -4 (1/4"SAE) you are increasing the volume to the calipers but are reducing the pressure to them."
This doesn't seem accurate. It's physics. Once those lines (consider it a closed vessel) are filled with fluid and free of any compressible component (air), once the pedal is applied, the pressure everywhere within that vessel is the same, no matter the diameter, no matter where you measure it. That's kind of a rule.
The only variable is a potential for larger diameter flexible lines, particularly stock hoses, to expand more than smaller ones, slightly increasing the amount of fluid that would need to be moved to make the same line pressure. But since at this point, the master piston has already advanced past the feed ports and closed them, no additional fluid can be introduced to make up for the swell of the hoses except more pedal travel to move more fluid.
After looking here: https://www.joesracing.com/master-cylinder-math/ I see I considered the acting slave piston area incorrectly, but the premise of maintaining the right master/slave piston size ratios is pretty obvious.
Enough for me on this subject.