4 wheel disc on 74 Duster - Prop valve?

Unless something is wrong, the caliper pistons should move outward in lock-step with the pedal. It is a simple volume-volume relationship. Have someone watch the rear calipers as you push the pedal. If it isn't leaking somewhere, the only other place the fluid volume can go is to move the "imbalance switch" piston, but that should be against the rock-solid front hydraulics. The calipers shouldn't retract much when you release the pedal, since all that retracts them is relaxation of the square O-ring seals.

Above assumes you have only fluid in the lines and your rubber hoses aren't bulging. Best guess is you have air in the system. Are the bleeds on the rear calipers at the very top?