Who has Dr. Diff 8 3/4 rear discs? Pt. 2

I have not read your other posts on this topic so I'm flying a bit blind here....
In 2014, I tried to switch to a manual setup. I have a '70 Charger with 12" Cordoba front rotors, 2.75 single piston calipers, the Dr Diff 10.7" rear disc kit and.......A '75 Dart power booster and 15/16" master cylinder.
I bought a 15/16" manual master cylinder from Dr Diff. It was terrible! Hard pedal but horribly inadequate braking. I also tried a few other master cylinders, two of them were over 1 inch in size and one was a factory iron 15/16" unit. ALL of them were bad, they all felt firm but ineffective. I gave up and put the power stuff back on.
It turns out, I stupidly had a disc/drum proportioning valve in the car! I switched to a drum/drum distribution block and braking improved. I have not yet tried to go back to manual brakes but the weight savings is tempting.