My first car was a 1972 Swinger , and it had the power disc brakes ( 318 / 904 combo driver train ) .
I constantly had problems with those brakes !!!
The pistons would come-out unevenly , thereby causing a pulsating brake pedal and premature pad wear .
Naturally , the rear drums would overcompensate for the lack of braking power on the front , and would lock-up easily
Good point. If the front rotors were turned to minimum or below and the linnings are worn, the added fluid cavities in those calipers would make a difference in front to rear responce. Only a larger bore master cylinder could overcome that condition. The perportion valve does what it does but has its limits.