On a good 4 wheel disc system Master Cylinder bore and caliper piston diameter come into play for optimal performance. You would be better to not use a proportioning valve and simply plumb in an adjustable one on the rear brakes. If you simply swapped out hoses to new from what you had before it sounds like there is still air trapped somewhere...
FWIW sometimes I open the bleeder and then have someone depress the pedal and then close the bleeder during the first attempt at bleeding. Kind of what Robert touched on earlier...