Well, this is interesting.

Let me approach differently. Why would the factory want a different pressure front to back when they can change brake "bias" by changing piston area, rotor diameter, drum diameter, width.

To have different pressure the pistons would have to be hard connected to each other. Maybe some are.

If in the master, the front and back reservoirs and pistons were totally independent of each other, the pedal would stop as soon as say, the rear flow stopped. Once hard in the rear, no flow would go to the front, hence no brakes up there.

Isn't it interesting that the brake system, is far more complex than meets the eye, eh?

Please correct me if I am wrong, I think a lot can be learned here.