Compatibility between power and manual disc/drum master cylinders
Bore depth for the push rod.
From 67 to 72, the bore diameter was different between power and manual, but not in the way you'd think was logical.
The manual brakes had a larger bore, which made for more pedal effort (1-1/16 or 1-1/32", I forget which). The power brakes came with a smaller bore, making for less pedal effort (15/16").
The problem with buying an MC today is that they don't necessarily list the bore diameter. And many sources don't even list a part for all applications (for example, 67 Barracuda disc, power or manual), even when they stock the identical part under other applications. It is generally recommended to get the 15/16" bore regardless -- this makes the pedal effort on manual brakes more in line with modern expectations.
All of these master cylinders (manual, power, disc, drum, A, B) are physically interchangeable (same 4 bolt pattern), but as he said, the pushrod will be different for power or manual, and there are at least 4 different bore sizes when you add in B-bodies.
The critical difference is between disc and drum. The disc MCs always have one chamber larger than the other, and the factory ones always have the wire bale closure. Drum MCs have 2 equal chambers, and many of them have a bolt-down lid.
