Well it could be a variety of things but in my experience usually it has to do with the prop valve (the one the brake light wire goes to). There is a valve inside that can slide one way or another and can cause some issues when bleeding a dry system. I would push fluid through the rear line until it comes out the rear bleeders, then do the front circuit and I would crack lines on the prop and metering valves to see if you can push fluid (air) out.
On a dry system sometimes it takes some creativity but you will get it done.
I recently had this happen on my 74 Charger. I didn't get fluid at the one rear drum. I cracked the new line where it goes into the wheel cylinder and had fluid there. The bleeder screw must have got some crud in it from somewhere and was plugged. I removed the bleeder screw and eventually unplugged that wheel cylinder bleeder passage. I would just loosen brake line joints to see if I have any brake fluid in each one.
As 70chall440 stated, if it is the rears that you are having the issue with it's almost always the proportioning valve tripped or stuck.
When there is a lack of fluid pressure in the rear the valve is designed to trip and direct the braking to the front. it is very easy to cause this issue while bleeding by pressing the brake pedal too far down on the initial bleeding.
I would try gravity bleeding the rears for a while and then stab the pedal a couple of times with all the bleeders closed. if you have enough fluid volume there it should reset. then you can carefully continue bleeding the system. The key is not to push the pedal all the way to the floor while bleeding the system.
I hope this helps you out.
If the master is bled, safety switch is centered and the rubber brake lines are good internally the FSM recommends pressure bleeding because manual bleeding has reduced fluid flow. I’ve had excellent results with the Motive pressure bleeder. Hope this helps.
Could be debris in one of the lines. Disconnect lines and put rag over the ends. Blow compressed air thought the lines to clear any blockages. If they blow clear and after you reconnect the lines and refill with fluid. Any blockages should be narrowed down.
Check the simplest thing first. Take the bleeder valves out and clean them out, making sure they're not clogged with rust and gunk. Most of the time this has happened to me it was because the bleeder valves were clogged with dirt and rust.