You said you're bleeding a completely dry system, right? Maybe the proportioning valve has the rear system blocked as a result of bleeding the front system first. Pressure on the front system with no pressure on the rear system results in the proportioning valve blocking off the "sensed" system with no pressure which, in this case, is the rear system.

Have you tried "re-setting" the proportioning valve to see if it's blocking off the rear brakes? That is; if you have a "proportioning valve" installed.

Or maybe the proportioning valve was blocking off the rear brakes before you started bleeding any of them.