I had a similar problem with a 70 Challenger that I once had. Upon braking, one of the wheels would lock up and the tire would skid making a loud screeching sound. The sound was coming from the right side and I was sure it was the right front wheel that was locking up. I bled the brakes, replaced the caliper, replaced the master cylinder and nothing seemed to cure it. I was going to replace the proportioning valve too but my friend suggested that I drive the car past him and brake while he observed from the sidewalk. I did so and he said that it was the right rear wheel that was locking up. Rear? I thought for sure it was the front. We then disassembled the right rear brake and discovered that the wheel cylinder had corrosion in it that was causing it to stick and lock up the wheel when braking. The point is, I suspect that it probably isn't your proportioning valve. Do you remember what color the brake fluid was that came out of your brakes when you bled them? Was it dark in color by chance? Brake fluid absorbs moisture and after sitting for 15 years I would suspect that you probably have some corrosion in one of your wheel cylinders or calipers and it's the source of your brake problems.
