Disc brakes require about 50% more line pressure than drum brakes. That is where the problems start.
 The factory used a number of methods to make the discs & drums work as a happy couple....Proportioning valve to limit pressure to rear brakes; changed the bore size of the rear wheel cyls; or both.
 One rear wheel locking. The 'locker' could actually be working correctly in this mismatched braking system; the non-locker could have grease on the linings, leaking/rusted up wheel cyl. Etc.
 The rear wheel cyl bore size needs to checked to see if it is correct for the PV & the car. Then the rear brakes need to be closely examined for mechanical problems that could cause lock up & considering the small &&&&&, new wheel cyls fitted.
 M/C bore needs to checked to see if it is correct for that disc/drum combo.
 One reservoir of the m/cyl is often larger than the other. For an enthusiasts car where oils/fluids get checked, it doesn't matter which chamber you use. Arrange according to your brake lines if you want too. It makes NO difference to the braking capability. Res line pressure valve was ONLY used with drum brakes, later deleted as unnecessary on later cars. Likewise the useless metering valve, just something else to fail. GM deleted these, not sure about Mopar.