If the brakes are stock go with the same size bore as stock, changing the bore size changes both pedal travel and effort needed to stop the truck.
If you are adding power assist … you on your own.
For my 46 1/2 ton with 54 (bendix) ‘non power assisted’ brakes 1” is the answer for stock and the new m/c. Using dot approved steel braided flex lines the pedal is solid, travel is right and effort right.
Larger bore than stock means less travel but more leg effort.
It is about psi, it’s the per square inch matters… the math gets long but it starts here
1” bore area is .79sq in, 1-1/8” area is .994.
The 1” bore multiplies force, 1-1/8 is a almost neutral multiplier
