Very soft pedal post brake pad replacement?

The vehicle listed above is a 1500 LTZ. Front brake pads need replacing. Back were done 6 months ago. Replaced the pads, the cylinders, the lines from the cylinders to the frame. Brake lines from there on are metal. Bled and tested. Hard brake with car off. Pedal goes to floor with car on. Very low braking and very unsafe. Bled numerous times. Same results. Bled all four brakes starting from the farthest to the closest. No change. Used 3 quarts of DOT 3 fluid with no change. Vacuum to power booster is 20 inches of Mercury. Check valve allows air one way but not the other way. This one has electronics connection to valve. No blown fuses. Master cylinder has not been drained past min level. No fluid leak at any location. Yes, the bleed valve is in the top position on the new calipers. Rotors are clean and plenty of life left. Brake pads are ceramic and clean. What else could it be? This all started after the new front brake pads were changed.