Jaguar XJ-S. Manual - part 108

 
 

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BRAKE CALIPER PARTS CLEANING:  Using the wrong solvent here could be bad, and water might not be too 
good an idea either.  Fortunately, any auto parts store sells a spray can of brake parts cleaner.  In fact, they often seem to 
offer two types of brake parts cleaner, often made by the same company; one that’s a basic cleaner, and the other is 
more environmentally-friendly or something.  They both seem to work pretty well, and they evaporate quickly without 
trace. 

 

BRAKE CALIPER ASSEMBLY:  FYI, the calipers in the XJ-S, both front and rear, correspond to illustration “D” on 
the generic page of directions that comes with the Lucas caliper rebuild kit.  The new seals go in either way, there is no 
inside or outside to them.  The groove itself has a tapered bottom, which tilts the inward edge of the seal up against the 
piston so it seals properly. 

The Haynes manual implies you should use only brake fluid when assembling pistons into calipers:  “Dip each piston in 
clean hydraulic fluid and enter it squarely into its cylinder.”  Many mechanics concur that the only substance that 
should be used when assembling pistons in brake calipers is brake fluid.  It doesn’t work too well, though; brake fluid 
tends to run right off of clean steel or stainless steel, leaving the assembly basically dry while you’re trying to slide the 
piston through seriously tight new seals. 

The ROM has a better idea: coat the seals and pistons with Girling disc brake lubricant.  Time was when a Girling 
rebuild kit came with a small packet of “red grease” for this purpose, but the modern Lucas rebuild kits do not include 
such a packet; you need to buy some separately.  Craig Sawyers says, “I have a tube of the stuff, and the data sheet for 
the different greases right here.  It is Girling PFG102, described in the table as: 

Rubber Grease (red), For use on rubber items in or near to DOT brake fluid. 

For use on hydraulic units in direct contact with brake fluid, providing extra protection 
against corrosion and ingress of dirt e.e. packing dust covers of master cylinders and brake 
piston dust covers. 

Warning - not other grease to be used in these areas.

 

“It is, well - dark cherry red and greasy!” 

Another tube of red grease was labelled “Genuine Castrol Girling Rubber Grease -- Specially prepared by Castrol for 
Girling”.  The only number on the tube is “Code 64947010”.  It might be worth noting that this tube appears quite old; 
Sawyers’ report above probably refers to a more recent product. 

If you can’t find the genuine Girling red grease, be very careful about substitutes.  There are substances intended to 
lubricate pistons in calipers during assembly, and there are substances intended to lubricate pads in calipers or the slides 
on floating calipers, and these are both commonly described as “brake caliper grease”.  However, the latter substance is 
usually black (it contains graphite) and will say on the package “Do not use on internal caliper parts.”  Chuck Sparks 
won’t even use packets of red grease if they’re not properly labelled from Girling; he’d rather use the stuff in the tube, 
since he knows that is the right stuff. 

It appears to be a common belief that silicone lubricant may be used for assembling pistons into calipers.  Nobody 
seems able to point to any official recommendation, though, and some mechanics suggest it is dangerous at best.  Jeb 
Boyd says, “I don't know if that would be from the same family as silicone brake fluid, but I do know that mixing 
silicone brake fluid with glycol based fluid produces a gelatinous mess that will trash a hydraulic system in a hurry.  I 
would steer clear of silicone grease...unless you are using silicone fluid.”  Play it safe and use the recommended red 
grease. 

Although the ROM advises coating the seal and the piston with grease, you might consider coating more than that.  If 
you coat the inside of the cylinder, it should help the piston slide into place without scoring.  And if you coat the lip 
area of the piston, it should help the boot slip into place better -- but be sure to remove any grease from the surface that 
contacts the pad, since you should be using a different substance there.  If you coat the areas of the piston and caliper 
under the boot but outside the cylinder seal, maybe it will help keep things from rusting, and perhaps catch any dust or 
rust scale that happens to get in there and prevent it from getting into the seal.  Finally, you might consider using this 
grease on the threads of fittings and bleeders. 

 
 

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Richard Dowling reports that there is one place you probably do not want to apply red grease.  “The shoulder around 
the bore, which takes the outer part of the boot, saved Girling a few cents by not having a groove machined onto it.  
Hence the boot is retained by friction between the rubber and the shoulder, with the large circular clip applying the 
force.  No groove means if you grease the boot it slides off very easily, and I doubt if it would stay put for long.”  It 
actually might not be a bad idea to apply some sort of sealant here to help the boot stay in place.  A sealant resistant 
to brake fluid is not necessary since it should never see any brake fluid, so any sealant that won’t attack the rubber 
of the boot will do. 

CRC makes a substance called “Disc Brake Quiet” that is red and comes in a 4-oz. plastic bottle.  This stuff is 
intended to be applied to the back side of brake pads immediately before installation, after which it dries to a 
rubbery glue that bonds the pads to the pistons.  I do not recommend the use of this stuff on the pads in the XJ-S, 
because it also bonds the pads to the boots -- and as Dowling says, those boots aren’t held down very well, so as the 
pads wear they can end up pulling the boots off the calipers.  However, it might make good sense to use this stuff 
when installing the boots!  Just apply a small bead around the shoulder before installing the boot with its clip. 

 

WARPED ROTORS:  The way to check for warped rotors is to mount a dial indicator on the upright or axle and 
position the tip against the friction surface of the rotor.  As you turn the rotor around, ideally the needle should stand 
still.  Moving less than 0.005” is good.  Big swings back and forth are bad, and will result in pulsing or shuddering 
brakes. 

Some people suggest that warped rotors are caused by tightening down the rotor attachment bolts or the wheel lug nuts 
too much.  This is hogwash, pure and simple, but if you don’t think so it’s still a simple matter to avoid the problem.  
Simply use a torque wrench to attach the rotor to the hub.  If you aren’t already using a torque wrench to mount the 
wheels, well, shame on you. 

Warped rotors are usually caused by defective rotors.  In the most obvious incarnation, the guys making the rotors 
machined them crooked.  Such mistakes should result in the rotors being returned for credit and a vow never to use that 
supplier again.  In a less obvious cause, the casting of the iron rotor itself prior to machining is faulty and results in 
residual stresses in the metal.  Then the rotor is machined straight, but the first time the rotor gets really hot these 
stresses relax, and the rotor warps.  It will shudder from then on.  If the warp is in the friction area of the rotor, you can 
merely have the rotor turned and it’ll completely fix it; the residual stresses are gone, it will never warp again.  
However, if the distortion included the hub mounting area, you’ll either need to remachine that as well or toss it and get 
a better rotor. 

Casting iron is not rocket science; it’s been done for thousands of years, and every casting company knows how to do it 
correctly to avoid residual stresses.  The problems occur when they put their profit margin ahead of quality control and 
start taking shortcuts in the casting process.  If you buy rotors that warp the first time you get them hot, do us all a favor 
and return them for credit.  Allowing companies to get away with making substandard products doesn’t help anybody. 

Yet another possibility involves rotors that are straight with no residual stresses, but have non-homogeneous 
metallurgy.  A little carbon here and there can make some portions of a cast iron rotor something closer to hard steel.  
Such hard areas not only have different coefficients of friction that result in pulsing brakes, but they also wear at 
different rates.  When one area wears faster than another, they gradually become effectively warped and start 
shuddering. 

It is possible to warp good rotors.  Typically, it requires getting them very hot and then letting them cool with an 
asymmetrical force or load on them.  That asymmetrical force can be gravity, if the car is parked rather than rolling; if 
you get the brakes really hot, make sure to drive a few miles to let them cool off prior to parking.  Another possible 
cause is a stuck caliper piston, which will first help get the rotor too hot in the first place and then hold a pad jammed 
against it while it cools. 

Of course, there are dumb things too.  If you leave the car parked for years, the rotors may rust everywhere except 
where the pads sit.  When the brakes are next applied, the rust is all sloughed off, leaving high spots where the pads 
were.  If parked in a flood-prone area, the rotors may rust on the bottom half! 

 

 
 

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ROTOR TURNING:  Rusty Beard says, “I took the front rotors off to get them turned.  I went to the NAPA parts store 
(a reliable U.S. chain) to get the rotors turned, and the parts clerk looked up the Jaguar in his book.  The book was 
footnoted that the rotors could not be turned, but only replaced.  Further, the minimum tolerances were not even printed, 
just left blank.  Thus, he would not do the work.  I went two doors south to a brake shop and that man had the book 
with all the minimum tolerances printed, and no further notes.  He did the job, and I got my car back together.” 

This problem probably stemmed from the bonded laminate rotors used at the rear; see page 437.  It shouldn’t relate to 
the front rotors, which are a conventional cast iron vented rotor design and can be turned like those on any other car.  In 
fact, it shouldn’t relate to the rear rotors if the original laminated rotors were replaced with solid rotors at some earlier 
time, which would be obvious because they don’t have the loose iron ring around the outer edge. 

 

BRAKE SQUEAL:  Dean Gosselin sends this tip:  “I have traced a major source of brake squeal on my 1991 XJ-S to a 
tire clean/shine product called "Formula 2000".  After 4-6 weeks of not using this product the brake squeal is nearly 
gone.  I traced this cause to the spray-on tire cleaning product after my father used the stuff on his Audi and it 
immediately resulted in major squeal under braking.” 

If you’re not using tire cleaners and still have a brake squeal problem, there are three distinct types of products available 
to address it.  One is a substance that you spray on the back side of the pads and allow to fully dry before installation.  
Typically this stuff is blue.  It dries to form a tacky layer that serves as a damper between pad and piston.  The second 
type is a red goo that comes in a 4-oz plastic bottle that is applied to the back side of the pad immediately before 
installation, and dries to bond the pad to the piston; as mentioned under BRAKE CALIPER ASSEMBLY above, this 
stuff is not recommended for the XJ-S.  The third type is an aluminum shim material that is cut to shape and stuck onto 
the back side of the pads. 

Some owners, however, report that the final fix for squealing brakes was to replace the pads with a different type.  
Notably, the organic pads tend not to squeal, while some types of semi-metallic pads squeal like crazy.  Since organic 
pads are undesirable from a performance and safety standpoint, there’s something to be said for insisting upon semi-
metallic pads from a reputable maker. 

 

DIRTY FRONT WHEELS:  Dust from the front brakes tends to get the wheels dirty, especially if you use them hard.  
There is a device available to prevent this problem.  It’s called KleenWheels, and it’s essentially a disk that seals the 
wheel on the inside so the dust can’t come out through the openings.  They are available at many auto parts stores.  Ed 
Avis reports that the no-name “turbo-vented dust shields” sold by J. C. Whitney, catalog number 78xx6976BF, are in 
fact genuine KleenWheels.  He also provides the info on the company itself: 

  KleenWheels 
 

 

5000 Oakes Road, Suite H 

 

 

Ft. Lauderdale, FL  33314 

  +1 

(954) 

791-9112 

It would be logical to assume that cutting off this airflow path would result in a reduction in brake cooling, and 
therefore poorer braking performance under severe, repeated braking.  However, most users don’t notice any detriment 
-- perhaps because they never use their brakes hard enough for overheating to be a problem.  The primary cooling 
airflow is from the inboard side of the brakes to the inside of the hub and outward through the vented disk.  The 
warmed air can then go either out through the wheel or inward under the car.  The use of KleenWheels prevents the 
first exit, so all the air goes inward.  So, the use of KleenWheels won’t reduce the airflow to the brakes, but it may 
cause more of the warmed air to recirculate back through the brake again.  Considering the airflow underneath the car 
during any activity where hot brakes are a concern, this is probably insignificant. 

 

BRAKE PERFORMANCE IMPROVEMENT:  Brakes will get hot; that cannot be avoided, since the entire point is to 
absorb the energy of the vehicle’s forward motion and convert it into heat.  The differences in how well brakes 
accomplish this generally result from A) how much mass there is to absorb the heat (only works once, though; the mass 

 
 

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won’t help the second time if it’s already hot), B) how well the brakes dissipate the heat they have absorbed to the 
surrounding air, and C) just how hot the brake components can get before problems occur.  In general, automotive 
brakes intended for street use are designed with enough mass in the iron rotors to absorb the energy of one stop, and 
then they are supposed to dissipate that heat to the air before the next stop.  If the rotors aren’t heavy enough, they’ll 
fade during the first stop.  If there isn’t enough cooling airflow, they’ll fade during subsequent stops.  You can improve 
brakes greatly by replacing parts such as pads and brake fluid with items that work at hotter temperatures, but 
eventually you will only be able to improve so much without either increasing the rotor mass or the cooling airflow.  
Since makers of road cars typically concern themselves with the first stop, it’s more likely that airflow is needed than 
additional mass. 

Remember that the amount of energy brakes must be able to dissipate rises with the square of the vehicle speed.  
Stopping a car from 120 mph requires dissipating four times the energy as stopping the same car from 60 mph.  
Slowing a car from 120 mph to 60 mph requires dissipating three times the energy as stopping from 60 mph.  If you 
plan on stopping from top speed, keep this in mind.  If you plan to soup up your car so the top speed is considerably 
higher than stock, take note.  Fortunately, when speeds are high is when there’s plenty of airflow around to help cool 
the brakes, so with careful airflow and duct design you can obtain a significant amount of brake cooling during the first 
stop. 

 

BRAKE BALANCE:  Ideally, the relative action of the front and rear brakes should be balanced properly so that when 
the brakes are applied hard enough to approach the limits of adhesion, the limits of adhesion at both ends are 
approached at nearly the same time.  Having one end lock up the tires while the other end is still not braking very hard 
is not good. 

In the simplest arrangements, such balance is achieved by careful design.  The designers of the car, hopefully using 
some actual road testing as a basis, design the front brakes and the rear brakes properly to obtain the correct balance.  
Typically, the diameters of the pistons in the calipers or the diameters of the pistons in the master cylinder are adjusted 
to obtain the desired balance.  This is the idea behind the design of the XJ-S brakes, at least in the author’s ’83. 

The racer’s method of accomplishing the same thing is to install two separate master cylinders side by side, one serving 
the rear brakes and one serving the front.  A beam connects the plungers of the two cylinders, and the pedal is 
connected to the center of this beam -- or a little off center.  By moving the pedal connection closer to one side or the 
other, the proportion of force applied to each master cylinder can be easily varied.  The driver takes a few laps and then 
gripes to the mechanic that one end or the other is locking up, and the mechanic adjusts the pedal connection 
accordingly. 

Once the proper brake balance is established, it is important not to screw it up by doing something silly like using 
different type pads on one end than the other.  The brakes in the XJ-S were designed for similar pad materials at both 
ends, so if you use metallic pads at one end and organic pads at the other, the balance will be a mess. 

Of course, the proper balance is not a fixed thing.  On dry pavement, heavy braking will result in a considerable shift of 
the weight forward onto the front wheels, endowing those tires with better traction.  So, the proper balance on dry 
pavement is with the front brakes doing considerably more than half of the braking.  But what happens in the wet?  Or 
worse, in snow or icy conditions?  In such cases of reduced traction, there will not be as much shift forward of the 
weight of the car, and proper balance would be closer to equal between front and rear.  But such a simple system as that 
found in the ’83 XJ-S will still provide the same balance as on dry pavement, so one could expect the front tires to lock 
up first under icy conditions. 

There are ways to deal with this.  In many automobiles, there is a device called a proportioning valve that is 
incorporated into the hydraulic line to the rear brakes.  When the brakes are applied lightly, as one would do in snow or 
ice, the full pressure from the master cylinder is applied to both the front and rear brakes.  When the brakes are applied 
more forcefully, however, such as when trying to stop quickly on dry pavement, the proportioning valve only permits a 
certain amount of pressure to the rear brakes and no more.  In such cars, it is appropriate to design the actual brake 
hardware with the balance a little stronger to the rear.  When applied lightly in snow or ice, the rear brakes will thus be 
stronger than the fronts, which is helpful in such conditions.  The proportioning valve then limits increased braking at 
the rear to avoid locking the rear wheels under hard braking in dry conditions.