Proportion valve

I believe the manual cars had the larger piston in the master. Seems odd Mopar would have done it that way. I would buy a NEW (not remanufactured) master cylinder anyhow. Go looking for the 15/16" bore. Also, if you replace your calipers, the 76 and later were larger bore. using these will shift your brake bias toward the front, as will smaller diameter rear wheel cylinders. You still need the combination valve from the disk car no matter what.