I think you would have to remove the master cylinder to measure the bore. I re-read the whole thread while trying to ignore all C-20, C-30, and longhorn info because those always came with power brakes and were plumbed differently.
Summary: According to the documentation Keith Seymore posted, manual and power both have a 1-1/8 bore size. I have heard before and seen posted on this thread the power master cylinder has a shallow push rod hole and and manual master cylinder has deep push rod hole. On C-10 the front reservoir is for front brakes, and for disc/drum setup the larger reservoir and smaller tubing size is for disc. So it looks like 54blackhornet got a good new master cylinder for C-10 manual disc/drum.
The next thing I would be interested to see is measurements and photos from actual C-10 71-72 trucks with and without power brakes. Measure and photo the pushrod clevis attach point to brake pedal lever pivot point that jocko mentions in post #55.
Somebody with factory power brakes on a 71 or 72 (C-10 only) measure this distance
And somebody with factory manual brakes on a 71 or 72 (C-10 only) measure this distance
I suspect they are the
same but would like to know and for the measurements to be documented. Like Steeveedee and BigBird05 mention these are just illustrations by humans. It would be great for measurements to be made, and only from 71 and 72 C-10s that are suspected to be original, not conversions of earlier years and not from trucks that have been converted from manual to power.