[Grim Reaper]

Here is a couple things to look at.

First let me explain how a twin master works.
The rear piston pushes the front. If there is air in the rear circuit then the rear piston will bottom out and push the front. In normal conditons the fluid bewteen the rear piston and the front is what pushes the front. This is why it it critical to bench bleed the master BEFORE installing it on the truck. If the master is not level air will be trapped in it and very hard to get out with normal bleeding.

Next problem. Did you install a Combination valve (Combination valve is the correct term, It has a proportioning valve for the rear circuit and a hold off valve for the front on Disc/drum set ups) for a dual master? If so they can be a booger to bleed. The inlets from the master are on the top and on most the rear circuit connection is on the side or on the bottom. When pump bleeding the air will compress and the Proportioning valve for the rear circuit will close and not allow fluid to make to the rear brakes.

That sounds like your problem. What I have done is cracking the line loose coming from the rear circuit from the master to the Combination valve. get it to where it's leaking. This will cause the master to gravity bleed so make sure it doesn't run out of fluid. Tap on the Combination valve with a wrench to try to dislodge any bubbles and they should burp out.

Tighten the line, refill the master. See if it will gravity bleed the rear circuit by just cracking open the rear bleeders.