I have been in your position a few times. All of the above is great advise. Look for the simple things, double/triple check to make sure your calipers are on the correct side. I had the this same issue on a B body and it was because I had to switch the calipers from the front of the rotor to the rear to clear the sway bar and I had the calipers on the wrong side. The bleeder was on top of the line but not on top of the caliper. I looked at those calipers for days before I switched them. I was chasing my tail and the problem was right in front of me. I would defenitly clamp the lines off, that will tell you if the air is in the front or the rear. As far as the prop valve, I have done it every witch way, In the Super Bee, I have the Drum/drum dist block and no prop valve and it works fine (no lock up) in my duster I have the drum/drum dist block and a adj prop valve and no lock up. The last thing you may want to look at is your rear wheel cylinders. I am not sure of what master you have or what wheel cylinder you have but in the disc master there is a take up valve inside it and on a drum master the take up valve is in the wheel cylinder. If you have two take up valves you will have rear wheel lock up. I would swap out the rear wheel cylinders to a disc/drum wheel cylinder. That could be causing the lock up. Hope this helps.