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The author’s ’83 has no such proportioning valves. However, Figure 9.11 in the Haynes manual shows such a device --
of course, the British can’t use conventional names so they call it a “Pressure Conscious Reducing Valve” or PCRV.
So, apparently some XJ-S’s or XJ12’s come fitted with such a device.
Yet another method to accomplish a similar benefit was used in two British sports cars owned by the author in years
past. Instead of having a brake booster between the pedal and the master cylinder as on the XJ-S, the pedal assembly
and master cylinder were the same arrangement as a non-boosted brake system. The Girling booster was an in-line
device -- pressurized fluid coming in one end resulted in boosted pressure coming out the other end. Neat, because it
could be mounted anywhere. But these boosters were mounted in the lines to the front brakes only. As a result, when
the brakes were applied lightly, the rears would come on strong first. But as more pedal pressure was applied, the
booster would increase the pressure to the front brakes.
Of course, modern cars all have ABS -- which doesn’t change a thing! It is still important to have a good balance on
your brakes, since you don’t want to lock up either end and get the ABS system involved before the other end is doing
its part.
Snow and ice aren’t the only conditions where the OEM brake balance might not be appropriate. If your car was
designed decades ago but you have mounted some modern super-sticky tires on it, you may be able to stop considerably
faster than the designers of the car expected you to. As a result, you will get more weight shift towards the front than
they planned for, and may end up in a situation where the rear tires are skidding while the fronts still have more traction
available. To optimize your braking system under such conditions, you would need to alter the balance to either reduce
the braking effect at the rear or increase it at the front.
It should be noted that proper brake balance is important for reasons other than avoiding wheelskid. When using the
brakes hard, as when going down a mountain road, the amount of braking done by each wheel is what determines how
hot each brake gets. If one end of the car isn’t doing its share of the braking, the other end is going to get hotter than it
should -- and possibly get into brake fade sooner than you would like.
When modifying or upgrading the braking system, many owners are justifiably concerned about effects on their brake
balance. Regarding the vented rear rotor upgrade described beginning on page 440, there is good news -- or bad news,
depending on your viewpoint. Since that upgrade uses the OEM caliper pistons to push the OEM pads against a rotor
that is the same diameter as the OEM rotor, it cannot possibly affect the brake balance of the car. The only way it could
is if the vented rotor were made of a significantly different material than the OEM rotor, but hopefully they are both
something close to basic cast iron. From a performance standpoint, the only thing the vented rotors accomplish is to
keep the brakes cooler so they can be used harder without fade. When used lightly, this upgrade should make no
difference at all. Of course, if your solid rotors are covered with oil and the vented rotors prevent oil from getting on
the friction surfaces, the upgrade will most certainly affect the brake balance -- putting it back where it was supposed to
be!
If you wish to alter the brake balance, you will need to either A) change calipers to something that has a different
effective piston area; or B) change rotor diameter. You could also change pad material, but most people concerned
with serious braking select pad material for the best fade resistance they can get rather than to adjust balance. You can
also change the size of the pads or the breadth of the swept area of the rotor (either change will typically require a
caliper replacement) but such changes are more likely to affect the ability of the brakes to absorb heat than to actually
affect brake balance.
CROSS-DRILLED ROTORS: There are three topics that should never be discussed in polite company: religion,
politics, and cross-drilled brake rotors. Talking about cross-drilling brake rotors always seems to start an argument.
There is little doubt that cross-drilling rotors makes them a bit lighter, but other than that everything is opinion. Brian
Schreurs says, “I race GM F-bodies at the club level (i.e., budget). Improved braking on a car with a reasonable system
to begin with is found almost entirely in the pad selection, not the rotors. Those in my club who have tried drilled or
slotted rotors have switched back to normal ventilated rotors. The surface tricks are prone to premature cracking and
shortening the life of the pads, with no appreciable benefit in braking to make up for it. Outfits such as Baer will try to
talk a customer out of buying drilled rotors if the car is intended for the track.