Brakes won’t ‘hold’

71 Demon. Have done the ‘73 disc brake swap including the disc brake master cylinder. It stops pretty well. I never drove it when it still had the drums so I can’t compare it to that, but it seems to stop just fine.

Earlier this month when I was at the car show/bracket race I noticed the brakes won’t hold the car for me to do a burnout. They hold just long enough to get a few rotations and then the car just pushes forward.

Any ideas of where to check first? Everything *looks* to be in good shape. Plenty of pad left, fluid level good etc.

Thanks.