Did you install later rear wheel cylinders?
If you use the older style rear cylinders you can have problems with getting the system bled. E-Burg wrote about it in Mopar Action a few times.
I had a real problem with soft pedal feel in my 74 Swinger that I had installed an aluminum master, untill I installed the rear wheel cylinders out of a late 70s pickup.
very solid pedal from that day on and no more rear wheel lock up.