Disc brakes feel spongy no matter how much I bleed them

My 67 ragtop Barracuda has a set of 73 front disc brakes and stock 10" rear brakes. I am also using a new manual brake master cylinder. I got the entire system off a donor car including the proportioning valve, lines etc. I also have new rubber brake lines. Yesterday, I took the master cylinder off and bench bled it with 2 short brake lines feeding over the top. It didn't help at all. The caliper bleeders are at the top and I've bled the system several times. I get poor braking, cannot lockup the wheels at all. Any suggestions?