Disc-Disc takes a different master cylinder then disc-drum, lots of folks ignore this when building a car figuring a master cylinder is a master cylinder. A proportioning valve is still the best way to be able to tune the brake bias from front to rear, Wilwood makes what's called a combination valve now. Used them on a few cars and IMO they are a super unit!

Could check some obvious things, are the front calipers being fed off the larger reservoir on the master cylinder? Are all the brake lines the same size, I've seen people mix and max sizes and use a bunch of hokey adapters, never gives good braking! As for bleeding, awhile back I got one of the new reverse bleeders----I'll never go back to the other style or trying to bleed with someone pumping the brakes and me cracking the bleeder! Also, look the lines over and make sure the routing is good, no pinched lines, and no big downhill loops in the line that go below the level of the master cylinder that will form an air trap you'll never get out of the system. With the brakes released, what is the clearance between the pads and the calipers? Some will also turn a rotor beyond it's prescribed limits in order to save a buck....