I had this problem once on my Corvette. Brakes worked fine until you got on the brakes at the end of the 1/4 mile pass. They released fine, but then half way down the cool down lanes to the pits, the front brakes would lock up. I had to back my push rod off to compensate for piston retraction. I thought at first it was BS, but it's been perfect for 2 years, and then I upgraded to hydraboost. Adjusted the rod in the same manner, and no issues.