Manual front Disc Brakes?

the proportioning valve reduces pressure to the rear brakes and the hold off valve incorporated in the combination valve delays pressure build to the rear so the front not only apply braking first but also supply about 70% of the total braking.That's why going to rear discs is not usually a big improvement...since the rear only supplies about 30% of your stopping power.
..yes discs require much more pressure (3500 - 4000 psi) that's why the pistons are 2 5/8" or 2 3/4" in the calliper but only 1" or 15/16" in the wheel cylinder.
get the proper master cylinder and a manual disc front / drum rear system will stop effortlessly with very little leg pressure
for a manual front disc / rear drum system most like no bigger than 1" or 15/16"master.
..a 15/16" master cylinder was stock for early A body manual disc front drum rear.
Put a 1 1/8" master on a manual front disc car and a lot of guys won't be able to lock them up.