Hard pedal

BTW, OP, if the MC you had was for drum/drum (which it sounds like) and had a residual valve on the front system, that that might be part of the hard piston retraction problem on the front calipers...... Too bad the MC is kaput.... you could try it again with the pressure off or a bleeder open.

One thing that is being forgot on the caliper fluid volume needed is that there is more retraction at times than just the few thousandths that the pistons seals pull back. Any runout in the rotors or bearing looseness will push the pads back a lot more, as well as any flexing in that are when cornering. (If I have dragging pads, I just makes a few hard turns and voila...they are free.)

This side play does not exist in drums (but there is drum runout) so just thinking about .005" seal retraction is not close to realistic. And, performance calipers are designed with a lot more seal retraction to avoid pad drag in racing. But this may not be the case for the OP's expected new rear calipers which are stock OEM types; it could have been the case for the Wilwood calipers in the thread I linked.