Here is a great answer from Mike Patrick from a few years back on the yahoo board. This is what I believe is true, and how I run my car. (mine is on RF)
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Many racers will put the brake proportioning/bias valve to the LF to
keep the wheel from locking up under hard braking. But this is often a crutch,
not a true solution, as the root cause of the locking/smoking LF is likely too
soft of a RF spring. Put a stiff enough RF on the car so it does not heel over
under hard braking, then you have both fronts providing braking capability which
is what the driver is wanting; otherwise you get a car pushing up the track
because the RF is braking and the LF is sliding.
Once you get the spring issues resolved, then you can
start to think about the turning enhancements that the brake proportioning/bias
valve can provide your car. If the LF brakes more than the RF, then the car
will turn or be pulled left under braking. On many tight bullrings, this is
what the driver wants; so the brake proportioning/bias valve would be installed
at the RF (remember the proportioning/bias valve can only take away braking, not
add). If the car pulls hard left during corner entry because you have a lot
of caster split, then you can balance this out by putting the proportioning/bias
valve on the LF. So the concept here is use large caster split so the car
naturally leads itself into the corner without driver input before the brakes
are applied, and then further control the lead in under braking (which will
likely increase the pull to the left) by adjusting the proportioning/bias valve
to give the desired amount of braking pull to the left. As the track changes
during the race, i.e., goes tight, then you could compensate some by
increasing the pull to the LF with more brake, or, if the track goes loose, then
you can reduce the pull to the left under braking and subsequently tighten up
the car under braking.
Either way, a brake proportioning/bias valve is an
expert's tuning tool and either should be left off a beginners car, or, turned
wide open so that the beginner driver spends his/her efforts to properly set up
their car, or, learn to drive it to the max of its capabilities before they
attempt to take advantage of the little extra that a brake proportioning/bias
valve might give their car. I know many, many successful Pro/Master Division
drivers who have never felt the need to tune the handling of their race cars via
the brake proportioning/bias valve.
