Maybe your packs are bad or a connection in the wiring is so bad that during breaking the voltage seen by the esc automatically gets detected and corrected by reducing the breaks. I know its a big stab in the dark but worth a shot! TBH, my tekin 632 3200 kv motor locks the wheels in my 4wd XXX-4 even with 20% brakes. Wierd huh?
Check out my custom converted 8ight:
Sub 7lb, lowest CG of any 1/8 buggy