master cyl.

Smaller MC bore will lessen the effort.
I use a 73 A-body power disc brake MC but run manually. I believe the bore is 15/16. I've also tried 1-1/32" and 1-1/8" bores.
The 1-1/8 bore gives a very hard pedal.
1-1/32 bore is livable.
15/16 feels best to most people.

You can measure your bore on the back where the push rod contacts the piston. (Maybe from inside the car)