Brake Issues..Need input

[ABX131]

My 63 has the running gear from a 76 4x4, including the axels and brakes. It has disk front and drum rear. We replaced the M/C and booster a while back with a remanufactured unit, we bench bled the M/C. The application is for a 68' Vette. We had the front rotors turned at the high school auto shop my son is in and they put new front pads in. The rear was good and needed no attention. We have bled the system and have no air in the lines. We have bled them a ton. The issue we are having is the brake pedal has too much travel and feel like they loose pressure and the truck will start to creep at stop lights. The pedal has a return spring?? Why would it need one? Shouldn't the pedal have enough pressure to push it self back? My son and I are having different theories as to what our problem is. He thinks the rotors are too thin explaining the long travel of the pedal. I think the M/C is not holding pressure thus allowing the truck to creep. Input please..