Rear brakes lock up... front disc

check to see that the brake shoes are installed properly, there are primary and secondary brake shoes the primarys have more lining on them and always go to the rear of the car, if installed impropperly ie: two primaries on one side two secondaries on the other, primaries installed in the front will cause this, if you added the disk brakes to this car without the kelsey hayes proportioning valve will also cause this, when converting to big bolt disk brakes you need the proportioning valve that goes with that setup and may also need the adjustable proportioning valve as the a bodies are light in the rear early a bodies are definately prone to this problem with shorter wheelbase and lighter overall weight......just sayin from experience, when dealing with brake systems it's better to test them to the fullest in controlled manner than to find out later that there is a problem, I raced stock cars so we had to know what the car would do to gain that advantage...actually a larger wheel cylinder on the rear will have less braking affect as more fluid required to move the pistons inside the wheel cylinder.....Just Sayin