No power drum brakes???

George Jets said
Manual brake cars (drum/drum) can benifit from switching to a 15/16" master cylinder.

This gives more leverage at the pedal and makes it feel more like power brakes with more control to boot.

Without the bulk of the power brake unit and the need for vacuum besides.

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Does the brake cylinder bore size have any impact? I see options for 13/16" and 15/16" bore size for the brake cylinders.

2nd question: Why are there different master cylinders for 9" and 10" brakes but the bore size is different. MC36338 says it is for 9" brakes....