[xj-s] Mysterious brake failure

XJ-S
1

This past weekend I took some friends on a road trip.
Absolutely no problems except one even though we went
through some of the hilly and hot terrain of BC’s interior.
On one particular long mountainous downhill stretch (Duffy
Lake Rd going west for those interested)I encountered a
mysterious brake failure. I had safely pulled over to the
soft shoulder looking for a small exit rd and felt something
strange in the brake pedal. Once stopped I discovered that I
had completely lost all braking. The pedal went right to the
floor and no amount of pumping would bring it back. Using
the emergency brake I pulled onto the small exit rd (which
happened to be to a lake) went for a swim and left the car
in the shade thinking that I had to let it cool before I
could investigate further. Lo and behold after about 45
minutes I discovered that the brakes had come back 100%. For
the life of me I can’t figure this one out. There is no loss
of fluid and no loss of braking power at all. I’m presuming
it is somehow related to the ABS/brake booster but I would
love some input because I can’t stand mysteries, especially
when it comes to brakes. Any insight would be appreciated.
Thanks.–
1994 XJ-S (XJ27) V12 convertible. #188832
Merville, BC, Canada
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2

Sounds like your brake fluid boiled. It does not take much temp to do that with old water laden fluid. Flush the system and you should be good. Check for any sticking pads from caliper problems not allowing the pads to release completely.

It takes about three liters of brake fluid to flush the system completely. Suck as much old fluid out of the reservoir first.

Geoff Green

95 XJS 4.0 conv, 76 XJ-S, 61 E-type OTS, 05 F250 Turbo Diesel, 10 Escape

Keeping track of every Jaguar XJ-S, with your help.
http://www.xjsdata.com

3.6 XJS Register paulzimmer@btopenworld.com

Eventer Register pascal.mathieu@perso.be

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3

Once stopped I discovered that I
had completely lost all braking. The pedal went right to the
floor and no amount of pumping would bring it back.
Lo and behold after about 45
minutes I discovered that the brakes had come back 100%.

1994 XJ-S (XJ27) V12 convertible. #188832

With the Teves ABS system, loss of the front brakes will result in
the loss of all brakes; you cannot apply pressure to the rear brakes
unless you can apply pressure to the front brakes. Of course, that
still leaves the question of how you could temporarily lose the front
brakes.

The most obvious explanation would be that the fluid boiled in the
caliper. It’ll do exactly what you experienced. Only you can tell
us if it’s actually possible that the front brakes got that hot.
Even with lousy fluid and other issues, it still requires getting the
brakes pretty hot, either riding them down a mountain road, driving
them very hard, or resting one foot on the brake while the other foot
is on the gas.

There are at least two factors that can contribute here. One is that
the fluid is old and full of moisture, which lowers its boiling
point. If it is regular Dot 4, it’s almost clear when new and
increasingly amber in color as it absorbs moisture. So perhaps a
fluid bleed is in order.

Another would be the use of the wrong fluid. Any good Dot 4 will
work. Dot 3 will not, and silicone fluid will reportedly result in
immediate brake failure as the pump turns the fluid to foam.

Jaguar recommends replacement of brake pads with half the lining
still intact. Reportedly this is so that the lining itself will
serve as thermal insulation to keep the brake heat out of the fluid.
So if your brake pads are thin, perhaps a pad change is in order.
Make sure that the pads on all four wheels are the same general type;
my preference is semi-metallic, others prefer ceramic of various
types, but the most important thing is that all four are alike.

Finally, it’s also possible that a valve inside the ABS valve box
jammed open, releasing pressure to the front brakes. Not entirely
sure how that would self-repair. However, if the valve jammed open
due to an electronic failure within the ECU, I guess anything is
possible.

– Kirbert

// please trim quoted text to context onlyOn 8 Aug 2012 at 11:15, Brummie wrote:

4

In reply to a message from Kirbert sent Thu 9 Aug 2012:

I’d say you simply cooked the brakes. Select 2nd in hilly
conditions to allow some engine braking and as said above,
flush brake fluid if it hasn’t been done before.–
Andrew. XJS project. http://jaguarxjsresto.blogspot.com.au/
Canberra, Australia
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5

In reply to a message from TyphoonXJS sent Thu 9 Aug 2012:

Same thing happened (was happening regularly) to me few
weeks ago. The car hadn’t been driven for a long time but
the fluid was brand new. It would only happen in city
driving where engine temps were above N.
Changed the water pump, thermostats, flushed the cooling
system (lots of mud came out) and the engine temps never
went above N for the past 1500 miles. And brakes never
failed. So I say look closer at your engine temps. Yes I
had the same weird pedal feel - like something is rubbing
somewhere before they would fail. A faint rub.

I will be rebuilding the calipers and actuator with new
seals and things soon though.–
'91 xjs v12 5.3 early facelift in BRG
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6

Same thing happened (was happening regularly) to me few
weeks ago. The car hadn’t been driven for a long time but
the fluid was brand new. It would only happen in city
driving where engine temps were above N.
Changed the water pump, thermostats, flushed the cooling
system (lots of mud came out) and the engine temps never
went above N for the past 1500 miles. And brakes never
failed. So I say look closer at your engine temps.

If engine temps would do it, I’d presume we’d be talking about a line
for the front brakes being close enough to a hot part of the engine
to get cooked. And it wouldn’t be an exhaust manifold, because those
get hot whether the engine is running hot or not. I’m not familiar
with the brake line routing on those newer cars, but presume a line
passes over the engine at the firewall. Maybe getting cooked there?
Still, does seem highly unlikely.

Having a hot engine compartment might easily be expected to adversely
affect the operation of the ABS pump. However, if that pump got hot
and quit, the symptoms would be different. Specifically, you’d lose
boost, so you’d still have front brakes but you’d have to push REALLY
hard to stop the car. You wouldn’t have rear brakes, but you
probably wouldn’t know that.

The one other explanation that comes to mind is the engine heat
affecting the ABS valve box somehow. Again, to lose ALL brakes in
this manner, you’d expect a failure in the front brake circuit. The
jamming open of the right valve in that valve body would do that.
Some sort of malfunction in the ECU controlling that valve might also
do it, but isn’t the ABS ECU located somewhere cool?

– Kirbert

// please trim quoted text to context onlyOn 9 Aug 2012 at 23:32, Pele wrote:

7

In reply to a message from Kirbert sent Fri 10 Aug 2012:

There is potentially another explaination. The pressure
switch on the pump could be failing. I had one
progressively fail, and had brakes then no brakes then
brakes then no brakes and then fluid running out of a crack
in the case of this switch, at the base.

-M–
Mike, 1990 5.3 XJS Conv., 5-speed, SE-ECU, TT Extractors
Lakewood, OH, United States
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8

There is potentially another explaination. The pressure
switch on the pump could be failing. I had one
progressively fail, and had brakes then no brakes then
brakes then no brakes and then fluid running out of a crack
in the case of this switch, at the base.

Did this result in the pedal falling to the floor? Because I would
expect the result to be a hard pedal that you’d have to press very
hard to get the car to stop, but it wouldn’t fall to the floor.

– Kirbert

// please trim quoted text to context onlyOn 11 Aug 2012 at 11:32, mike90 wrote:

9

In reply to a message from Kirbert sent Fri 10 Aug 2012:

How about under-bonnet temps being high enough for the
brake fluid to reach a certain temperature in the plastic
vessel that’s high enough for it to start boiling soon
after some pressure is applied or it enters the (hot)
calipers a few times?–
The original message included these comments:

If engine temps would do it, I’d presume we’d be talking about a line
for the front brakes being close enough to a hot part of the engine
to get cooked. And it wouldn’t be an exhaust manifold, because those

–
'91 xjs v12 5.3 early facelift in BRG
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10

How about under-bonnet temps being high enough for the
brake fluid to reach a certain temperature in the plastic
vessel that’s high enough for it to start boiling soon
after some pressure is applied or it enters the (hot)
calipers a few times?

That fluid doesn’t go to the calipers. The lines to the calipers are
deadheaded, the fluid in the lines and in the calipers themselves
never moves other than a miniscule back-and-forth as pressure is
applied and released. That’s why it’s a good idea to bleed the
brakes every now and again to get that old fluid changed.

Now, the fluid in the reservoir does run around a lot on the ABS
cars, namely from reservoir to pump to valve body and back to
reservoir. If that fluid got hot enough, perhaps the pumping action
would heat it enough in addition to cause it to foam. If so, this
could definitely cause the problem, as those bubbles getting into the
master cylinder would wreak havoc. Thing is, though, that’s a really
big “if”. The boiling point of Valvoline Synthetic brake fluid is
311°F. I would think that if the temps got that hot under the hood
that the rubber parts would be melting and the paint would be peeling
off the hood. And the plastic reservoir would probably melt.

– Kirbert

// please trim quoted text to context onlyOn 13 Aug 2012 at 1:08, Pele wrote:

11

In reply to a message from Kirbert sent Mon 13 Aug 2012:

Ok, so how about this - the under-bonnet temps pre-heat the brake
fluid enough and then something else (probably all that city
braking) finishes if off and the pedal goes to the metal?
How else would one explain that ever since I changed the water pump
and thermostats and generally overhauled my cooling and ignition
systems the under-bonnet (as well as coolant, always sits in the
middle, sometimes falls towards C a bit on motorways) temperature
went down considerably and in the 3000-odd miles I never had a
single brake failure again?–
'91 xjs v12 5.3 early facelift in BRG
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12

In reply to a message from Pele sent Tue 21 Aug 2012:

check for sticking caliper pistons caus the brakes to bind and
boiling the fluid, that what caused mine to fail in the same
situations.–
1984 5.3 HE xjs!!! croydon where airports were invented
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13

In reply to a message from Pele sent Tue 21 Aug 2012:

check for sticking caliper pistons caused the brakes to bind and
boiling the fluid, that what caused mine to fail in the same
situations.–
1984 5.3 HE xjs!!! croydon where airports were invented
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14

Ok, so how about this - the under-bonnet temps pre-heat the brake
fluid enough and then something else (probably all that city braking)
finishes if off and the pedal goes to the metal?

On an ABS car, maybe. Presumably the city braking will cause the ABS
pump to run more, and a pump heats fluid. Of course, if true I’d
chalk this up as another reason I’d prefer never to own an ABS car.

How else would one
explain that ever since I changed the water pump and thermostats and
generally overhauled my cooling and ignition systems the under-bonnet
(as well as coolant, always sits in the middle, sometimes falls
towards C a bit on motorways) temperature went down considerably and
in the 3000-odd miles I never had a single brake failure again?

Dunno any other explanation, actually, unless you accidentally bumped
something that fixed the problem!

– Kirbert

// please trim quoted text to context onlyOn 21 Aug 2012 at 0:00, Pele wrote:

15

check for sticking caliper pistons caus the brakes to bind and
boiling the fluid, that what caused mine to fail in the same
situations.

This makes sense, but doesn’t explain how correcting engine
overheating might make the problem go away. A hot engine doesn’t
significantly affect brake caliper binding, AFAIK.

– Kirbert

// please trim quoted text to context onlyOn 21 Aug 2012 at 0:23, 647w wrote:

16

In reply to a message from Kirbert sent Tue 21 Aug 2012:

in all honesty I cant see engine temperature affecting brake fluid
unless it was on fire. hope you find the fault and its an easy fix–
1984 5.3 HE xjs!!! croydon where airports were invented
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17

In reply to a message from 647w sent Tue 21 Aug 2012:

I spoke too soon - just now after an hour or so of city
driving my temperature was 2-3 needle widths past the middle
and the bonnet was hot. I am not liking this. New fan clutch
for me I guess?–
'91 xjs v12 5.3 early facelift in BRG
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18

Continuing the discussion from [xj-s] Mysterious brake failure:

I know this is a late response for this thread, but I had the same thing. Recent brake master cylinder rebuild and brake fluid flush. Driving normally with brakes working fine. Stopped for about 5 minutes, got back in and foot to the floor, no loss of fluid, very strange. Within 5 minutes, everything was 100% firm. Didn’t happen again for about a month, it’s either full brakes or no brakes, not a confidence builder.
I stripped the calipers and replaced the caliper seals, brake discs and pads all around. Inside the calipers, the piston seals looked glazed as did the bores of the brake piston walls. Since this has been completed, no further issues. It is quite bizarre but the only reasonable explanation is that the old seals fail in the contaminated calipers if they get hot. There is a lot of crap in the calipers that doesn’t come out with a regular fluid flush.

19

Martin, I am curious which brake system is on your car?
Vac booster, Teves III or Teves IV?

Edit: I see your profile says 79 XJS so I assume vac booster…

Regards
Bob

20

yes, it does not have ABS. The other thought I had on this would be to check the seals in the master cylinder. the seal kit is not expensive and very easy to fit.