Brake bleeding problems....help!

[Mr Twister]

Hi Everyone
I hope everyone is staying safe in these trying times.
I've been working on my 72 4x4 brakes. I Installed new rear cylinders, new lines to the cross over lines at the front bumper, new proportioning valve & a new master cylinder.
I bench primed the master cylinder, added the proportioning valve and primed that with the master cylinder attached. Connected all the brake lines. I removed the switch from the proportioning valve and screwed in a home made stop to keep the cylinder centered.
The front brakes bled out without any problems. No matter how much fluid I pull through the lines at the back wheels I keep getting bubbles. I've went through the process a few different times now and still cant bleed the back wheels.
I'm at a complete loss and could sure use some help or advice here.
Thank you!
Mr Twister