Hydratech Brake boost

[jlipman]

I am new to the board. I recently purchased a barn find. It's a 72 C10 that has been parked for 10 years. The PO did a lot of work on this truck. It has a 383, lowered, bagged and quite a few 1/2 completed installs.
One of them is the Hydratech Brake assist. I'll tell you what I know to see if that sheds any light on what is going on. I checked all of the lines and they are plumbed correctly. I bled the master cylinder in place. I attempted to bleed the brakes using various methods. I tried using a bleeder that pressurized the lines at the master cylinder. I also tried bleeding with the unit that attaches to the bleed nipple on the drum and pulls the fluid. I cant get fluid to the rear drums.
Also, my brake light is on. When the engine is started and I apply the brake and release it, it does not return to the original position.
Any help would be appreciated!