Trying to order a new master cylinder for my power disc brake setup off of a 74 dart sport. Summit offers two bore diameters, one is a .938, other is 1.032. Anyone know which is right for the power discs?

I'd check RockAuto.com they have great details for looking up parts.

pretty sure they were 1.125" bore. i've got a new one if you need it.

'73-'76 A-bodies equipped with power disc brakes originally came with a 15/16" bore master cylinder.

That'd be the .938" one he mentioned. Doc would there be an advantage (or disadvantage) to him going with the 1.038" one? I know on a manual non power setup a larger bore is less travel/more pedal effort/likely needed with 4 wheel discs to fill the calipers but how does that play out in a manual brake app? (going with a larger bore)

I've got the 1-1/32" (1.031") MC with powerbrakes on my '73 Dart.
Even got slightly larger diameter front calipers (C-body). 10" drums in the rear.

I would say the 1-1/32" MC is adequate for daily usage, but not for performance braking. With good tires it's pretty much impossible to lock the front wheels.

Got the exact same disc/drum brake system on my '62 NY wagon, but with a 15/16" MC and its brake-performance is way better.