[Chevy nutcase]

I had the exact same problem on my 66 when I replaced everything including a new dual reservoir master cylinder. The front brakes bled instantly but even with a friend pumping I couldn't get a drop to the rear brakes. Bench bled it a second time and still nothing. I only had one idea to let gravity help so I opened the rear bleeders and disconnected the hardline from the master cylinder. Luckily I had a length of clear tubing big enough to fit tightly over the flared end of the line I removed from the master cylinder. I then filled a bottle that resembled a ketchup bottle with brake fluid, stuck the hose to it and hung it from my open hood like an IV bottle. It took a minute or two till it started running out of the bleeders but I then closed the bleeders and hooked the line back to the master cylinder and voila I was able to bleed out the remaining air bubbles like normal. Sounds janky but it worked.