Need Help!!

I have a 1968 plymouth barracuda. 318. Manual brakes. I put a new master cylinder on. Bench bled it before install. New brake lines. Tried bleeding and broke bleeder. Installed new brake cylinder on right rear. No fluid at all checked other side nothing. Check front just for the hell of it and have perfect fluid bled perfect. Let gravity feed over night and nothing. Used a vacuum and got some fluid barely any. Pulled off brake line and had perfect fluid coming out of line. Installed line and tried to bleed by pumping and vacuum and nothing. Any ideas? Does have one new and one older wheel cylinder on rear. In need of desperate help!!