Braking force distribution

[NoHachi]

Sadly, a pedal box is not perfectly adjustable. Look at the above curve, it has a knee in it. As you brake harder, more weight is transferred to the front of the car. This means you need proportionally less pressure going to the rear and more to the front. For every amount of decelleration G's the ratio is different.
That's why you use one of those bias valve knick nacks. It flows 50%-50% in normal conditions (taking into account piston area) and above a certain pressure (which correlates with decelleration Gs) a valve pops open and less pressure is fed to the rears as the rears get lighter. The knee in the curve approximates the gradual decrease you need.
If the knee is too high you get one spot in the curve where the rears lock up, but braking might be fine at higher pressures. Very tricky to set up just perfect and not many bother.

Like this: