Hey Thumper, you are correct, your pedal should go soft when bleeding the rear discs.
Sounds like a blockage in the line going back, or possibly a MC problem.
Did you replace/rebuild the MC as well? I had a problem years ago after rebuilding my MC, due to incorrect rebuild parts.
As for portioning valve, this allows you to adjust the pressure between the front & rear for proper brake bias.
The issue of unequal braking pressure can be caused by different sized caliper pistons - front & rear, as well as brake pad size.
You should get what you have working first before you think about a proportioning valve. You may not need one.