Jim, I'm having the same issues with my '73 Dart.
Front discs (11") and rear axle upgraded from the wimpy 7-1/4 to a 8-3/4" with 10" drums.
I don't know if the 8-3/4 has larger brake cylinders or not, but the rears lock up 'fairly easy' with my car aswell. Stock prop.valve.
I describe it pretty...