hello group! Im kinda stumped here and am seeking help... I've been doing brakes as a backyard mechanic my whole life and have never run into this problem. this 73 challenger has been sitting about 30 years when I got my hands on it... upon restoring it, we've come to the brakes. We've got new calipers, hoses, pads, shoes, wheel cylinders, and master cylinder. I adjusted the rear shoes, bench bled the master, then bled it at the line fittings, then bled the entire system starting from the furthest bleed point to the closest...with not the greatest results, meaning the pedal never feels rock solid. I've been very careful to not over extend the master cylinder by not letting the brake pedal reach the floor, or let the fluid level get too low. I then bled the entire system again with the same result. I then removed the master cylinder again to make sure it was still bled and it was, so I reinstalled it and bled the entire system again to the tune of 2 large bottles of dot3 fluid with the same result, no solid pedal. what am I missing? I dont have a manual for this car, but I read in my van manual that if you have made repairs to the brake system, centralizing the pressure differential valve is necessary... not sure what that is but on youtube I found a guy saying that if you dont stabilize the proportion valve, you'll never be able to properly bleed the brakes. it requires a special tool that holds the valve in place during the bleeding process. has anyone had any similar experiences? all and any help greatly appreciated!