[Tommy]

A few more thoughts: As you have two brake master cylinders, the front and rear systems should not affect each other. Therefore, the rubber brake lines on the rears should not affect the action of the front brakes.

You mentioned going to smaller diameter master cylinders to increase the pressure in the lines. The price for increasing the pressure is that you decreased the volume of fluid going to the calipers. Is it possible you are (and were) running out of master cylinder throw before the pistons are fully extended? When you bleed the brakes, compare the motion of the brake pedal when the bleeders are open (i.e., full throw on the master cylinder) with the motion when the system is bled and sealed. If the pedal moves the same distance both times, then your master cylinder is too small.