To tune the Brake Balance, you need the Brake Controller available from the Tuning Shop. Once installed you can change the split between front and rear brake strength.
Front/Rear Balance: Numerically the range is from -5 to +5 range of adjustment. In practise this is a relative adjustment number away from the baseline brake balance that each car has. We never get to know an actual percentage brake balance number that we would often see in other sims and real life such as 62% Front Brake Bias.
More Forward Brake Balance will promote more understeer when braking.
More Rearward Brake Balance will promote more oversteer when braking.
This Brake Balance can also be adjusted while driving using the MFD menus.