It really does not matter how the system is supposed to work. What really matters is the practical application in real life and at least two of us have experienced the real life example of virtually no breaking power as a result of a ruptured rear brake line. The fluid drains out of the reservoir and there is no fluid left in the system to provide the means of stopping the vehicle.
I took professional automotive repair classes back in the day and I remember learning that things are supposed to work that way.
I also learned how brake systems are designed with safety measures to prevent total loss of braking when a line ruptures. I was quite surprised when those theorem were not applied in practical application when my brake line ruptured.
