unhooking the brake line at the master does nothing to indicate that fluid pressure is being passed to the rear calipers. It just shows the master is making pressure.
What you should do is loosen the brake line fitting at the calipers BEFORE the flex line to see if you get pressure there. Once you do, tighten it down and check it at the caliper end.
Chances are the old flex hoses are collapsed and blocked.
Fact is stranger than Fiction