[Quick Toy]

Is the pedal still spongy, or just not holding the rear brakes enough?
If it is still spongy, sometimes a "bench bleed" of the master cylinder helps to get some of the air out.

What are the rear calipers off of?

I'm not sure about your setup but, have you adjusted the rear brakes, to take up most of the slack between the piston/pad and the rotor? My setup is self adjusting, and works with the "E" brake, but I preset it by adjusting the pistons first. (The heads screw in and out of the pistons)

Paul