FWIW...I had the same issue. 4-piston K-H disc's up front with drums in the rear. No amount of bleeding helped...pedal always went to floor even though I was getting no aur out of the rears. Changed the rear wheel cylinders to new and....bingo, I quickly got a hard pedal. Not sure if the rear cyls were sucking air on pedal release or what but the new ones solved the issue.