Jaguar XJ-S. Service manual — part 109

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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.

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“We (being my club) regard drilled and slotted rotors as appearance items only, harmless on the street but worthless
and bordering on dangerous on the track. Our formula is: race compound pads (or an aggressive street pad, depending
on application), steel braided lines (for pedal feel and protection from debris), ATE Super Blue fluid (resists boiling),
stock ventilated rotors (durable and cheap), and whenever possible, ducting.”

Others, of course, argue that cross-drilling rotors helps clear dust from the face of the pads during heavy braking and
thereby improves braking. Still others argue that the objective is to clear gasses that are generated by the friction. And
proponents, of course, claim that their brakes were vastly improved after drilling, and point out that many exoticars
come with drilled rotors.

Whichever, cross-drilling should perhaps be reserved for solid rotors such as the OEM rears on the XJ-S. Christopher
Riek says, “Do not cross-drill your ventilated brakes! Doing so allows air to be sucked in through the holes and would
destroy the effectiveness of the venting. It's like putting a squirrel-cage blower in a chicken-wire housing -- it spins fast
but doesn't blow much air.”

Henry Fok adds, “When some motorcycles switched from solid rotors to crossdrilled (without changing anything else),
brake pad life decreased by half.

“It is also thought, in some quarters, that crossdrilling can eventually lead to worse brake performance if the drilled
holes load up with debris.”

GROOVED ROTORS: Providing grooves across the surface of a brake rotor supposedly has similar objectives to
cross-drilling, but with less tendency to cause cracking. However, it has the minor disadvantage that the benefits (if
any) vary with wear and resurfacing; as the rotor gets thinner, the grooves get shallower. It has the signficant
advantage, though, that it could conceivably be done to vented rotors without affecting the airflow through the internal
passages.

BRAKE UPGRADE HARDWARE: For brake upgrade stuff, you can check AP Racing at

http://www.apracing.com/car/index.htm

or SPV Racing at

http://www.spvracing.com/

or Harvey Bailey Engineering (page 711), or Rob Beere (page 712).

Master Cylinder/Reservoir/Booster (non-ABS)

BRAKE FLUID RESERVOIR CONNECTION HOSE: To make the brake fluid reservoir fit under the hood, Jaguar
uses a remote reservoir connected with metal lines and short pieces of flexible hose. If these hoses need replacing, do
not use conventional fuel line; the brake fluid attacks the fuel line material, and the result will be darkened fluid with
lots of crud in it.

The fact is, none of the hose commonly sold at an auto parts store will serve. Also, nylon tubing is unsuitable; the
brake fluid soaks right through it, and then it hardens and shrinks.

Of course, using Jaguar original hose is not recommended, as it is a British non-metallic product. After all, you are
now having to replace it. But there are other cars that also use hose to connect a remote reservoir to the master cylinder,
including Mercedes, BMW and Fiat, so you can check other dealers for suitable hose. Andrew Weinberg says,
“Volkswagen Beetles also use a remote reservoir, and the hose stocked for that application works perfectly... The hose
is available from any VW supply shop. The only hitch is it’s a (nice?) blue color.”

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Non-automotive hose is also a possibility. You can visit an industrial supply store and attempt to find a clerk who
knows his stuff, or a reference manual that lists acceptable applications for different hose materials. Commercially
available air hose works well in this application; it is red with a black inner lining, and is sold in many hardware stores.
It’s so cheap that many Jaguar owners can’t believe it could ever work. The inner lining is EPDM, which is
compatible with brake fluid. Note that this hose will not withstand the pressures seen in other parts of a brake system,
such as the lines from the chassis out to the brake calipers on the suspension; air hose can only be used on non-pressure
portions of the brake system such as the reservoir connections.

Keep in mind that this application sees no pressure, so high-pressure brake or hydraulic hose is unnecessary and will
only make it very difficult to install over the plastic fittings. Some suggest that you use lines that will hold perhaps 30
psi in case you ever want to “pressure bleed” the system, but just about any hose will handle 30 psi.

While you’re replacing the hoses, you might consider tossing the original metal tubes and simply run the new hoses all
the way from the reservoir to the master cylinder. This makes for a cleaner appearance under the hood. You need to
take care, however, that the engine shaking on its mounts doesn’t cause the corner of the air filter housing to rub on the
hoses and eventually cut a hole in them. If it did, the reservoir would drain and the warning light would come on in the
dash before there were any operational problems with the brakes, but you still wouldn’t be happy with brake fluid all
over the engine compartment peeling all the paint off the chassis.

BRAKE FLUID RESERVOIR LOCATION: A brake fluid reservoir must be located above the master cylinder so that
fluid will gravity-feed into the master cylinder and air bubbles in the master cylinder will rise into the reservoir.
Unfortunately, the remote reservoir location on the pre-ABS XJ-S is stretching the definition of “above”. It works OK
when kept topped up, but if you ever have air in the hoses or master cylinder for whatever reason it is suggested that
you unbolt the reservoir from its support bracket and position it about six inches higher, merely allowing the hoses
connecting it to the master cylinder to flex. With it in this position, operate the brake pedal a few times. You might
also just wiggle it a little. With any luck at all, any air in the hoses or in the master cylinder will bubble up into the
reservoir. Once all the air is cleared, you can bolt the reservoir back onto its bracket.

BRAKE FLUID LEVEL SWITCH: Hey, it’s electrical, so it’s discussed on page 594.

BRAKE FLUID RESERVOIR CAP FAILURES: Apparently the cap occasionally breaks into a center portion and an
outer portion. According to Michael Neal, “This is a problem and tends to reoccur if you don’t modify the reservoir.
This is on the pre-abs XJ-S brake reservoirs. Usually just flattening the sharp vertical ridge on the reservoir with a file
will do it.”

Better fix: replace the entire reservoir with one from a Mitsubishi. See page 594.

MASTER CYLINDER REBUILDING: There was an “early” type master cylinder used before 1977½, and a “later”
type used up until the advent of ABS brakes. However, apparently not all of those “later” types were the same. In the
Jaguar manual section 70.30.09, step 14, a seal is installed on the secondary piston with the lip facing forward. In step
15, a second seal is installed in a second groove facing rearward. Chapter 9, section 7, steps 13 and 14 in the Haynes
manual say essentially the same thing. However, some master cylinders only have one seal on the rear end of the
secondary piston rather than two. The one in the first step is the one that was omitted; if only a single seal groove is
available, the lip on the seal should be installed facing rearwards. Of course, if you have purchased overhaul kit
#M7673, you will have one seal left over.

POWER BRAKE SERVO: The (non-ABS) servo assembly comes apart in the center, by twisting one half until the
retaining tangs line up with the cutouts. There is a special tool for this job, but Bruce Hayes reports the tool can be

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improvised. He carefully mounted the master cylinder in a vice, and fashioned a tool to attach to the studs on the
housing to turn it.

Front Brakes

FRONT BRAKE PAD GROOVES: Some of the pads offered for the front of the XJ/XJ-S have grooves across them.
Ferodo pads, for example, have two parallel grooves across the face of each pad the short way; each groove is about
1/8” wide, and they are about 1-1/16” apart. These grooves might help with cooling, or getting dust out from between
the pads and the rotor, or letting water get out on rainy days, but Chuck Sparks says the primary objective is to prevent
squealing; the grooves break up the long, solid surface of the pad into three separate shorter panels.

If you get pads without grooves, you might opt to cut your own grooves. Brake pad facing material is not difficult to
cut; a hacksaw will do it. The Ferodo pads are not grooved all the way to the backing plate; the grooves stop about
1/16” short of the base plate.

FRONT CALIPER REMOVAL: The front brake caliper is attached to the hub carrier with two bolts. The steering
lever is likewise attached to the hub carrier with two bolts. And, just to make things interesting, one of those bolts is
shared; there are a total of three bolts, two short and one long. Some of these parts are clearly illustrated in the ROM:
the hub carrier (also called a stub axle carrier) is shown at the beginning of Section 60 under “Accidental Damage”.
The steering lever is shown in Section 57.65.01. The way the three components come together is shown as clearly as
can be expected in Section 70.55.02 or Section 57.55.29.

When removing the long bolt, don’t lose the shim that falls out. See below.

The two short bolts are the same length. However, the short bolt holding the upper end of the caliper to the upright has
a smooth shank and safety wire holes in the head. The short bolt holding the forward end of the steering arm to the
upright looks like a regular high-strength bolt, threaded to the head and no wire holes. I guess brakes are considered
critical and steering isn’t.

Removing two bolts will get the caliper loose -- but won’t get it off. There is also a hard brake line wrapped in a
semicircle between the outer end of the brake hose and the port on the lower end of the caliper, and this line is firmly
attached to a bracket that is attached with the two bolts holding the steering arm. Hence, if your objective in being in
here is not to rebuild or replace the caliper you will want to remove all three bolts in order to get the caliper to come off
with its hard brake line attached. Otherwise, you’d need to disconnect the line and get involved in brake fluid. By
unbolting the bracket you can hang the caliper by a wire to avoid stressing the hose and just bolt it back on when done.

Remember to turn the steering back and forth while trying to remove and reinstall the forward steering arm bolt.
Otherwise, the shock absorber is in the way.

FRONT BRAKE CALIPER SHIMMING: As mentioned above, there is a shim (or a couple of shims) between the
steering arm and the brake caliper at the long bolt. The objective of this shim is to compensate for variations in the
thickness of the caliper mounting lug to avoid stressing the steering lever as the bolts are tightened. If your plan is to
take things apart and put them back together with the same hub carrier, caliper, and steering lever, you need to take care
not to drop or lose those shims when you take the long bolt out. If you put it back together with the same shims in
place, you should be OK.

If you do plan to replace something -- exchanging the caliper for a rebuilt, for example -- you will need to measure for
the correct shim thickness. Mount the steering lever with its short bolt tight and the long bolt threaded in but not
tightened; you may want to fit a flat washer on the short bolt between the dust shield and the steering arm to make up