When I put disk brakes on my '49 half-ton back in the 80's, I used a Nova/Camaro disk/drum master cylinder.
The '75 factory service manual lists 2 master cylinder bore sizes for that application, 1.125" for power brakes and 1.000" for manual brakes.
I used a manual master cylinder like this one
Non-Power master cylinder on my truck.
To see how much of a difference there is between the two cylinders I did this calculation.
Area of 1.125" circle = .995 sq in
Area of 1.000" circle = .785 sq in
Pedal force of 50 lbs times mechanical advantage of the pedal (7) -> 50x7 = 350 lbs.
Power cylinder: 350 lbs / .995 sq in = 352 lbs/sq in line pressure
Manual cylinder: 350 lbs / .785 sq. in. = 446 lbs/sqin line pressure
446/352 x 100 = 126%
Bottom line, the smaller bore gives you a 26% increase in brake pressure for the same pedal push.
