No brake pressure and the brake pedal goes to the floor

I have replaced a part of the brake line on my explorer. I have bled the lines and had no issues. The brake pedal was back to normal. Then I go to start the vehicle and my brake pedal goes to the floor. What else might be the problem?

I had a damaged front caliper driver side and all the brake fluid leaked. I just finished replacing front rotors, calipers, and brake pads for both sides. After installing the new parts, I bled the brakes both sides. If you pump brake paddel once or twice when engine is turned off, the paddel gets hard as it would be under normal circusmatance. But, when engine is running and you press the brake paddel, it sinks to the floor without any pressure. The vehicle doesn't stop and brakes don't work. I have bled the brakes twice and there is not air in the system (as I know). What am I doing wrong? I have changed master cylinder and brake calipers on other vehicles (Nissan Maxima 1987, Pontiac Sunbird 1983) before without any problems. Can you please help me? Thanks.

:D
Well I didn't get any help but I solved the problems. Wanted to post back to forums so it might help someone like me. The problem was that I had already mounted the new calipers on the Explorer before "bench bleeding" them. Even though after install I had let the gravity run fluid thru calipers and then bled the system as you would normally do, there was air traped on the top corner of the calipers because the bleed nipple is on the bottom side of the calipers.
Solution. I uninstalled the calipers. Turnered them upside down with the bleed nipple being the highest point on the calipers and let the gravity do its work. A lot of traped air came out. Once clear fluid was filled in the plastic tubing, I tightened the nipple. Instaleed the calipers and then bled the system again. Worked perfect.