Difference between a drum brake master and disc brake master cylinder; disc brake master has a larger chamber for the front brakes and small chamber for the rear. A drum brake master with have equal size chambers. Disc brake setups; the front disc does more of the stopping duties for the car with an appropriate proportioning valve.
By the way, the early 1970 disc brake Challengers had a valve under the driver's seat to regulate the rear brakes; about mid-year they integrated the valve with the other brake components up by the master cylinder and deleted the under seat valve. Basically at the time, going from a two piece brake line front to back to a one piece brake line.