Even if the check valve is good, which mine is, if you depress the brakes before your burnout you use what vacuum there is in the booster. Therefore, while doing a burnout the car goes into boost and no vacuum is created in the booster. While you roll out of the box, still under acceleration, and try to depress the brake pedal to stop, vacuum has not yet restored back in the booster and this can be what causes a stiff pedal. That IS why my brakes do not work after a burnout and why I went with a line lock(easier on the rear brakes also) . Other than the burnout , I do not have anyother problems. A stiff pedal at the end of a race could possibly be a leaky valve.