Brake expert help needed!!!!

[bcj67chevy]

I have a 1972 chevy truck w/ power brakes, disc up front and drums in the rear. Only had the truck for a couple weeks, brakes were terrible.

First of all when the truck is off the pedal is rock hard form the get go and only goes down a couple inches.

Changed the rear brakes: shoes, wheel cylinders, all hardware, and brake lines from rear hose above the diff. (front pads and rotors looked good visualy)

Tried to bleed the rear brakes and can't get it to work, try to pump up the brakes but the pedal only goes down about two inches before it gets rock hard(from the very first pump), then when i hold it down while the bleeder is broke loose the pedal doesn't go down and barely any fluid comes out.
If i do the same thing with the front i get the same hard pedal but then the pedal does go down. Could only do the one front because the bleeder broke on other side.

I even tried to bleed it directly at the master cylinder, and same thing.

Is something messed up with the front that it isn't letting the rear work??
What would cause the rear brakes to not bleed??

Any advise would be appreciated.
Thanks,
Bryan