Looks like this thread has been sitting for a bit, but I have a similar problem. '76 CB750K. I just rebuilt the front caliper (purchased new "caliper A" with new piston, seal, pads, bleeder screw, and new brake tubing). I bled the brake, using a piece of clear plastic tubing attached to the bleeder to watch for air and fluid. Pumped the brake lever until it was hard, held it down, opened the bleeder, watched air and fluid come out, closed the bleeder, released the lever. Kept the reservoir full. Did that many times, until no more air bubbles came out. Now I pull the lever and I get about two soft squeezes in before it firms up, and even then it doesn't seem to be applying much pressure to the rotor. Is it possible I didn't finish bleeding? Should I try the reverse pump method or syringe?
Another thought, the replacement piston was hollow and I didn't put a cap on it. So it's a bit shorter than the original piston. Is it possible that the new piston is too short to grab the rotor? Seems like it would be forced against the rotor regardless. Any thoughts welcome. Thanks!