Which master cylinder bore size to use for power disc Kelsey/Hayes

If all the pistons aren't firmly seated against pads, with the pads in full contact with the discs, there can be voids holding air.
Lotsa folks bleed till "blue", lol, cuz it takes a buncha pumping to get all 4/8 pistons in total contact with the pad, and more pumping to then move the pads out to disc.
You may not have got all the air out, and pads etc not all the way out when you bled before. Try bleeding it now, and make sure a pad isn't hung up and not sliding into contact with disc.
I've had a pad in wrong, kinda cockeyed, and the piston was bending the pad instead of applying braking, - giving a poor pedal.
If you have a seized piston, common on k/hs, I used to pull the pads and carefully twist the piston in the caliper, with water pump pliers to check them.
Just musing, good luck.