could be a bad master cylinder, thats about the only thing that comes to mind since you have already pretty much covered all the other possibilities.The two hydraulic cylinders in the calipers I believe have only one bleeder valve. Also when you bleed your brake system do you thoroughly bleed them out?? I would go buy 2 big bottles of fluid and bleed each line till you get clear fluid coming out that way you know its all knew fluid and no chance of air in the lines. If that doesnt work then change the master cylinder.