Sorry but if you've clamped off the rear lines and it still doesn't work you've got air in them.

It's the massive surface area that the rear calipers require that ruins the pedal feel.

Clamping them off diagonally won't do anything, try clamping the fronts off and seeing what that does, because then thats effectively the same as two standard front calipers. If the pedal still hits the floor then, you've got a problem.