I had a similar problem with a rear disc conversion. I have a pressure bleeder and ran about 2 qts. of brake fluid trying to get the pedal firm with no luck. I worked for a week every night trying to figure out why the brakes wouldn't pump up. as a last ditch measure I took off the calipers and put the bleeders at 12 o clock (they were at about 10 o clock on the brackets). I bled them again in that position and it did the trick. I would have bet a months pay that they were fully bled with my pressure bleeder but there was air trapped in the system. I think someone already said it could be the issue. I would give it a shot before replacing anything. FYI... Make sure you put a clamp on the piston before you bleed the caliper off the rotor. Your piton will pop out otherwise. Don't ask me how I know that...