Unbalanced brakes
Stock Mopars frequently are rear brake biased. Nothing has to be wrong with the system for the rears to lock up first, it’s probably factory stock.
Changing the master cylinder will have no effect on the front/rear brake bias.
Changing the rear wheel cylinder bore will change the front rear bias, and is an option if you can find a different wheel cylinder bore.
Adding an adjustable proportioning valve is probably an easier way to adjust the bias, because there aren’t a ton of available wheel cylinder bores widely available anymore
Effectively, I'll try to go to the simple stuff first :)
Is this a proportional valve and can it be defective?
It's original and is 53 y.o.
View attachment 1716317796
That is a factory proportioning valve, you can tell because the line nuts are different diameters.
They can be defective, but again just because the rears locked up first doesn’t mean the prop valve is defective. That was a pretty common situation with factory Mopars, especially the disk/drum cars.
Front/rear bias is also changed if you run a staggered tire set up, or have changed the front/rear ride height. Running with the front low and rear of the car high (fairly common muscle car stance) takes weight off the rear and makes it more likely to lock up the rears first.