Having trouble bleeding rear brakes
 

So I'm new here to the forum and I am having some problems bleeding my rear brakes. Its a 69 Camaro and it had factory disc/drum brakes. I decided to upgrade them and swap the drums for brakes. I upgraded the brakes to factory 2010+ Camaro Brembos.

What is equipped:

1. Wilwood 1.125" master cylinder
2. dual diaphragm booster
3. SSBC manual proportioning valve
4. 14" 6/4 pot brembo calipers
5. Factory steel lines
6. Rubber rear hoses
7. Stainless Steel front lines

I've been trying to bleed the rear brakes for the last few weeks but I can't get any pressure for them to lock up. So far I've tried:

1. Bench bleeding the master cylinder twice
2. Lashed the master cylinder and pushrod
3. Gravity bled the system
4. Bled the brakes starting at RR, LR, RF, LF
5. Vacuum bleed all the calipers
6. Bled each rear caliper starting with the inside bleeder, then outside
7. Tied both rear calipers together and vacuum bled them
8. re-bled them with the proportioning valve wide open
9. replaced a questionable steel lines

I've moved over a quart of fluid through the rear calipers and I am still battling a pressure issue. I don't think its an air lock.
I think the factory steel drum brake line is not allowing it to build pressure or something else?
I need some help/guidance to what could be the problem.

https://cimg5.ibsrv.net/gimg/camarof...05fe06b1e5.jpg