[xj] Brake lacking full power assist

XJ
1

I think I may have a Brake Servo failing, but looking for
opinions. Suddenly my power assist is not all there. Brakes
function and have checked fluid level etc. Looking for thoughts
here as to trouble shooting etc. Once previously a careless mech
knocked off the vacuum hose to the servo and brakes were almost non-
existant. Does not seem to be case here but acts as if not all
there either. But I have checked for vacuum at the hose and full
vacuum is there as expected.

Doug Grace–
84 XJ6 Black/Black
Douglas, AZ, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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2

In reply to a message from Douglas G. sent Fri 23 Nov 2012:

Doug:

Haven’t heard from you in a spell. Good in one way of looking at
it. not so good in the other. Like now, the Jaguar isn’t purring!!

If the booster is completely dodo, brake effort is formidable. So,
is it harder than nusual or is it that the car doesn’t brake as
well? The former points to a leaking diaphram in the booster
cannister. The latter suggest either the front or rear brakes are
not responding. Or in another way of putting it, a hydraulic fault,
rather than a vacum fault.

With the engine idling to assure maximum vacum, apply the brake
pedal. if it goes down, it maight be a hyudraulic fault or vacum
leak.

So, using a minivac, remove the caum supply hose and apply a
vacumn. See if the cannister will hold a vacum or if you are even
able to establish one.

And, then look csarefully for evidence of leaking hydraulic flud at
the master cylinder andat each of the calipers.

I assume, (that bad word), that the fluid resevoir is topped off.

I was going to respond earlier, but my typing skills on the laptop
are much worse than on this desktop. That means realy bad.
And, the laptop is having a hissy. It lost the wireless connection,
why, I dunno and it is late, so for another day.

My new companion is right out the door here. he prefers to lay on
the csrpet as opposed to the hardwood floor. Smart, those Cocker
Spaniels. He is a ten year old that I got from a rescue shelter.
he was slated for… at the dog pound.

Carl
CArl–
The original message included these comments:

opinions. Suddenly my power assist is not all there. Brakes
function and have checked fluid level etc. Looking for thoughts
here as to trouble shooting etc. Once previously a careless mech
knocked off the vacuum hose to the servo and brakes were almost non-
existant. Does not seem to be case here but acts as if not all
there either. But I have checked for vacuum at the hose and full
vacuum is there as expected.

–
Carl Hutchins 1983 Jaguar XJ6 with LT1 and 1994 Jeep Grand
Walnut Creek, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

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FAQs: http://www.jag-lovers.org/xjlovers/xjfaq/index.html
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3

In reply to a message from cadjag sent Fri 23 Nov 2012:

Yes it has been nice to have a little freedom from the forum, in
re: problems etc. I do read it regualarly, since I never know when
I might learn something which happens on a regular basis.
Insofar as the brake thing, I recall how hard it was to brake when
the vacuum hose was off once before. This seems to be somewhere in
between the mushy feel of failing hydraulics. My experience has
been with a partially failing wheel cylinder or leaking fluid you
could pump the brakes and at least momentarily get a pedal again.
In this case, it seems to be a permanent condition in the fact that
I can brake with success and with no pulling in either direction,
it is just MUCH more difficult and I can feel the mechanical aspect
taking place, but their is no loss of pedal.
Hope this gives a better explanation.
I have sent David B., an email and awaiting a reply. In the
meantime I am going to pull the front wheels on Monday and check
the conditons there.

Doug Grace–
The original message included these comments:

it. not so good in the other. Like now, the Jaguar isn’t purring!!
If the booster is completely dodo, brake effort is formidable. So,
is it harder than nusual or is it that the car doesn’t brake as
well? The former points to a leaking diaphram in the booster
cannister. The latter suggest either the front or rear brakes are
not responding. Or in another way of putting it, a hydraulic fault,
rather than a vacum fault.

–
84 XJ6 Black/Black
Douglas, AZ, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

===================================================
The archives and FAQ will answer many queries on the XJ series…
FAQs: http://www.jag-lovers.org/xjlovers/xjfaq/index.html
Archives: Jag-lovers Forums - Jag-lovers

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4

I think I may have a Brake Servo failing, but looking for
opinions. Suddenly my power assist is not all there. Brakes
function and have checked fluid level etc. Looking for thoughts
here as to trouble shooting etc. Once previously a careless mech
knocked off the vacuum hose to the servo and brakes were almost non-
existant. Does not seem to be case here but acts as if not all
there either. But I have checked for vacuum at the hose and full
vacuum is there as expected.

A characteristic symptom of a failed booster is that the brake pedal becomes
very hard, Doug - and superhuman strength is required for any braking
effect…

With a failed brake circuit, pedal travel increases substantially - half way
down, and more pedal force is required for the same effect as there is
braking on only one axle. So which is it…?

The booster seldom fails partially, though a leak will interfere with
operation - and a leak will be evident as constant hissing as long as the
pedal is held down. During normal operation you may hear a brief ‘hiss’ as
pedal is applied - this is air entering the booster through a channel
coaxial with the pedal rod to the booster/master cylinder…

The principle is that while driving the ‘two chambers’ in the booster are
both at engine vacuum. Applying pedal; air is let into one chamber and the
higher pressure effects the assist, and with constant pedal the booster
diaphragm moves - closing the air inlet, thus maintaining the applied
pressure according to pedal force…

As said; two tests are relevant; with the engine idling listen for telltale
hissing, with the pedal released or constant pedal positions there should be
none. Then, with the engine shut down, repeatedly operate the brake pedal;
the pedal should become gradually firmer and higher as vacuum is ‘used up’.
If the pedal immediately becomes hard with the engine shut off; the booster
is indeed leaking. Tests will only show if there is a booster leak, of
course - or if the one-way valve has failed…

…which really should have little effect on braking with the engine
running. The valves primary function is to retain the highest vacuum
generated under various conditions; at low idle in gear a defective valve
may cause perceptible loss of booster assist…seldom encountered with a
moving car…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)-----Original Message-----
From: Douglas G.
Sent: Saturday, November 24, 2012 12:33 PM
To: xj@jag-lovers.org
Subject: [xj] Brake lacking full power assist

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5

In reply to a message from Douglas G. sent Fri 23 Nov 2012:

Doug, I agree with Frank that a failed booster will cause a
hissing sound when the brake pedal is depressed. The pedal
will feel very, very hard. When mine went out I had to
continue using the car until the replacement arrived. My
right leg got a very good workout in the meantime!–
The original message included these comments:

I think I may have a Brake Servo failing, but looking for
opinions. Suddenly my power assist is not all there. Brakes

–
Bruce Hugo '80 XJ6 SIII
St. Helens, OR, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

===================================================
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6

Doug,

is your problem permanent or intermittent? I once had the problem that
the booster worked fine at most times, but sometimes failed. It turned
out to be a very slight leak of one of the vacuum hoses allowing
sufficient vacuum to build up to drive the booster at any revs above
idle, but neutralising vacuum at around 1500 RPM. A worse story I’ve
been told is that some mechanics fail to install vacuum hoses allowing
them to collapse with the same effect, but typically at high vaccuum
operation.

Good luck, but don’t play too much with your life - no booster is almost
like no brakes on a two ton car with a hydraulic setup designed for
power assist …

Jochen

75 XJ6L 4.2 auto (UK spec)> Date: Sat, 24 Nov 2012 19:44:31 +1300

From: "Frank Andersen"franksue@xtra.co.nz
Subject: Re: [xj] Brake lacking full power assist

  • -----Original Message-----
    From: Douglas G.
    Sent: Saturday, November 24, 2012 12:33 PM
    To: xj@jag-lovers.org
    Subject: [xj] Brake lacking full power assist

I think I may have a Brake Servo failing, but looking for
opinions. Suddenly my power assist is not all there. Brakes
function and have checked fluid level etc. Looking for thoughts
here as to trouble shooting etc. Once previously a careless mech
knocked off the vacuum hose to the servo and brakes were almost non-
existant. Does not seem to be case here but acts as if not all
there either. But I have checked for vacuum at the hose and full
vacuum is there as expected.

===================================================
The archives and FAQ will answer many queries on the XJ series…
FAQs: http://www.jag-lovers.org/xjlovers/xjfaq/index.html
Archives: Jag-lovers Forums - Jag-lovers

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7

In reply to a message from =?ISO-8859-15?Q?Jochen_Gl=F6ckner? sent Sun 25 Nov 2012:

It appears that the booster is OK, but still not sure. Need to
check out the brakes on a hoist and not available until Monday.
Thanks to all.

Doug Grace–
The original message included these comments:

is your problem permanent or intermittent? I once had the problem that
the booster worked fine at most times, but sometimes failed. It turned

–
84 XJ6 Black/Black
Douglas, AZ, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

===================================================
The archives and FAQ will answer many queries on the XJ series…
FAQs: http://www.jag-lovers.org/xjlovers/xjfaq/index.html
Archives: Jag-lovers Forums - Jag-lovers

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