[ERA 626]

Brake Bias, This is often used while racing if the track conditions change. For street use set it and leave it. I race Shifter Karts, we often use the brake bias on different tracks. I will do several hot laps and let the tries and brakes come up to temperature. Then coming into the pits (in a safe place) I will stand on the brakes. This will tell me several things, does it pull to one side? (air in one caliper) or does the rear lock up? does the front lock up? (bad)
To test it go to a safe place! do a HARD brake at about 20mph. Experience what the car does, does the rear lock up early and cause a spin? or does the front lock up? If it is set up correctly you should have a little more rear bias then front. If the fronts lock up, this will do bad things while heavy braking coming into a turn.

All this knob does is restrict the amount of fluid passing through it ie proportioning valve (brake bias)