1995 Chevrolet Silverado • V8 4WD Automatic • 160,000 miles

I have a 95 chevy pickup k1500. I replaced the master cylinder, power booster, and bled the brake lines numerous times and my brakes still go to the floor. If I pump them up a few times they work like normal but then right after letting off for a second they go right down to the floor when pushing on them again. What is the problem here?
May 28, 2011.

If there's no external leak anywhere -double check MC could be leaking internally

May 28, 2011.
I plugged both outlits on the master cyliner and pumped brakes. The pedal gets hard so I believe there is no leak internally. Any other suggestions? Could the ABS control module have anything to do with this?

May 29, 2011.