97 GMC Sierra brake problem

I stopped at a sign yesterday and the brake pedal suddenly went from standard height to near the floor like the first stage of the master cyl failed. I bought a new master cylinder, installed it and bled the whole system. It IS NOT back to normal. It stops. With the engine off, the pedal it solid, no fade, but running, after a stop if you push harder and steady, the pedal goes down. I thought the booster was bad, but no vacuum leak, no idiot lights on. Is there some part of the ABS system that would cause something like this? I like good, solid brakes, now I'm confused. Thanks. :runaround: