I had this same exact problem on my 88 when I got it. It had been sitting about 18 years. New rubber lines, rebuilt calipers. No change. Swapped the master cylinder and that fixed it. I looked at the old master and the equalization port was plugged up. Its a tiny port that allows the presure to bleed off when you release the pedal. In my case, the more you used the brake the tighter they got. I think I could have just cleaned it out and reused the master, as it wasnt leaking.
In reality, my problem was both flexible lines and the master from sitting too long. What was messing me up, was I could get good fluid and pedal when bleeding. Then it would get pressure after a drive. Personally, I had never had this type of problem with a master cylinder. HTH
