I don't know of any mopar using a three bolt attachment setup, what is the source for that Mancini unit? For the era in discussion the OEM M/C rear port feeds the front brakes, period. You cannot use what happens when bench bleeding the M/C as an assumption of what happens installed. Hydraulic pressure is what makes the internals all move when in operation, something you do not really have when you are bench bleeding a master. So you have to bottom out the rear piston before the front piston will move and that is mechanically done, not hydraulically.

Since you don't tell us the application for the master cylinders in question any further help is guess work.