It may be that warning switch is not resetting or you may still have air somewhere in the system. If air gets in at the master cylindar, it has a long way to go to get out. I replaced my MC and had to bleed twice. Are you following the rather complicated bleeding sequence for the front calipers? I got an EZBleed tool which uses air pressure from a tire and it has worked much better than pumping the brakes. It hasn't happened to me but other have said that pumping the bkakes to bleed sometimes ruins the seals on the MC piston.