Went through the same hassle and did allot of head scratching.

I had the 4 piston calipers on the fronts and they held in the burnout box easy. Then I changed to the single piston calipers for the spindle mount wheels and it would push forward on the burnout.

I have to make sure I pump the brakes and trap at least 1000 psi on the line lock and it will hold .
I have a 3/4 master cylinder on the front and a 7/8 master cylinder on the back running dual 4 piston calipers