using a factory dual reservoir master cylinder (don't know piston diameter). All the flex lines are new.
Has the RPV (if equipped) been removed from the rear circuit?
Yeah, swapped it out for a distribution block when I did conversion.
The RPV "Residual Pressure Valve" is often in the Master Cylinder in the port behind the brake line tube fitting.
Since you mentioned that you did the explorer rear brakes and the shoes in the hub look good then you should not have pistons that need to be retracted manually. The screw in pistons (manually retracted) are for the parking brake in the caliper assembly. Not the shoe in the hub type you have.
I have had new out of the box brake lines be bad and that would be the first thing I would check.
Since it is both the rear brakes it is probably the rear hose, the RPV, the proportioning valve, a kink/pinch in the brake line or something like that is causing the pressure "Stick" in the rear line.
If you can get a pressure gauge that will fit the bleeder port you can check to see if the front and rear pressures both drop properly and if you have a small amount of pressure causing the rear to drag.
