Manual disc brakes vs power disc brakes questions

15/16"=.937"

15/16" was the standard size for A-bodies with 9" drums and power disks. It was a common bore size across quite a few mopar platforms as well.

As far as "working properly", that's not the right way to put it. A-bodies work just fine with either a 1-1/32" bore m/c or a 15/16" bore master cylinder, manual or power. The difference is completely down to pedal feel and effort. The only reason the factory used one vs the other was a subjective decision about how the brakes should "feel" with each system. Either one will stop the car just fine.

Just buy an aluminum m/c from Dr. Diff. It's $95 for the 1-1/32" bore master and that includes the 4 to 2 bolt adaptor. It's an extra $30 if you want the 15/16" bore because Cass actually sleeves down the later m/c's from 1-1/32" to 15/16". Having used both, I prefer the 15/16" bore and that's why I pay more to have them. But that's just my opinion.

Mopar Aluminum Master Cylinder Kit