In reply to a message from Craigy sent Fri 17 Jun 2005:
That does not sound good. Get a long piece of heater hose or
something similar, put one end to your ear and hunt around for the
leak with the other. If you can hear it just standing there you
should be able to locate it more precisely in a few minutes.
Then, if it is rubber hose, take a pair of vise grip pliers and
clamp off the vacumn hose from the manifold. You can drive and
brake fine without the servo boost and you can adjust your
carburators in peace and quiet, a deal with the servo later.
As to the servo, if the leak is in a hose, replace that, if it is
in the servo you willhave to take it out and get it rebuilt.
There is a diaphram in the big end of the servo, early models had a
rigid plate with a seal around the edge (leather first, rubber
later) and later models had a flexible rubber piece. I would look
there first. The end of the servo screws off, you need special
churchill tool Number (X?X?X) or else a piece of plywood with 3
holes drilled into it to fit over the mountng bolts on the big end
of the servo. A little penetrating oil and maybe tweaking the
cover will let you get it open without too much trouble.
good luck, let us know how it worked out.
P.–
Peter J. Smith, 1966 3.8S, 67 MGB
carson city nevada, United States
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In reply to a message from carsoncitysmith sent Fri 17 Jun 2005:
Hi Peter,
I’ll try that!
Would I not notice a reduction in braking performance if there was
a serious air-leak?
In the meantime many thanks for your help.
Craig–
The original message included these comments:
Then, if it is rubber hose, take a pair of vise grip pliers and
clamp off the vacumn hose from the manifold. You can drive and
brake fine without the servo boost and you can adjust your
In reply to a message from Craigy sent Sat 18 Jun 2005:
Hi I have seen the vacuum bottle getting a hole in it from rust but
you have to find it first with the hose. pajtas–
jaguarjoe 54 XK 120 rdstr 1961 MKIX 94 XJ6
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In reply to a message from Craigy sent Sat 18 Jun 2005:
Craig,
With that size of port vacuum leak, you would probably see a
serious reduction in engine performance long before you even
had a chance to apply the brakes.
I’ve been running around with a dysfunctional (read:
capped-off) servo for about a year now, and I’ve actually
gotten used to the non-powered brakes. I’ll eventually get
around to having the servo rebuilt, but in the mean time, I
figure I’ll be OK just as long as Bambie doesn’t jump out in
front of me when I’m doing 80 on the Interstate, and as long
as I remember that I’m not driving the Jag when I hit the
brakes on the DeVille.
Jacques–
The original message included these comments:
Would I not notice a reduction in braking performance if there was
a serious air-leak?
In reply to a message from Jacques B. sent Sat 18 Jun 2005:
Thanks Guys,
I had another play with it the other evening, and it seems that the
noise may only be coming from the servo vent/filter in the engine
bay. This sound seems to resonate from below the car with the
bonnet / hood shut.
I took the filter off and put my thumb over the pipe. This resulted
in a fairly strong vacuum and a drop in engine speed. I couldn’t
hear any other air noises.
So do I presume that all is OK and just play with the carbs a
little longer.
On that note, they seem to be a bugger to get right. I thought I
had the mixture right then to find that after about 30 seconds idle
it over fuelled / spluttered / black smoke. I then weakened the
mixture which seems to make the idle less stable.
Are you referring to the “mushroom” air filter that says “AC” on it and is
connected to the brake servo? There shouldn’t be any air flowing into that
hose except for a pulse when you put on the brakes. If you get a continuous
flow then your servo is causing a vacuum leak and you’ll never get your
carbs adjusted correctly.
I had another play with it the other evening, and it seems that the
noise may only be coming from the servo vent/filter in the engine
bay. This sound seems to resonate from below the car with the
bonnet / hood shut.
I took the filter off and put my thumb over the pipe. This resulted
in a fairly strong vacuum and a drop in engine speed. I couldn’t
hear any other air noises.
So do I presume that all is OK and just play with the carbs a
little longer.
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In reply to a message from Craigy sent Tue 21 Jun 2005:
Hello Craig,
There should be no vacuum in the servo filter hose when the brake
are off. When you step on the brake, the hydraulic pressure causes
the air control valve to open and let air pressure in to drive the
servo piston. The air valve should close when you release the
brakes. If you have constant vaccum at the filter, your servo is
leaking. You will never get the carbs set up properly if you are
leaking vacuum through the servo.
Paul–
The original message included these comments:
I took the filter off and put my thumb over the pipe. This resulted
in a fairly strong vacuum and a drop in engine speed. I couldn’t
hear any other air noises.
So do I presume that all is OK and just play with the carbs a
little longer.
Craig
In reply to a message from PS sent Tue 21 Jun 2005:
OH GREAT!!
Do you think it may be the valve that’s stuck or the servo itself.
I’m sure that the servo was refurbshed about 3 years ago (in fact
looking at my parts catalogue, servo repair kits were bought by the
previous owner for both the fluid & Vacuum sides.
What do I need to strip down to check?
Thanks in advance
Craig–
The original message included these comments:
brakes. If you have constant vaccum at the filter, your servo is
leaking. You will never get the carbs set up properly if you are
leaking vacuum through the servo.
In reply to a message from Craigy sent Tue 21 Jun 2005:
Sorry Guys,
Just another thought.
Seeing as the braking performance is as good as ever - does this
suggest that the actual servo is working ok but the non return
valve (or what ever it is called ) is not?
Please excuse my ignorance as I have never had to strip the braking
system before.
Cheers
Craig–
The original message included these comments:
brakes. If you have constant vaccum at the filter, your servo is
leaking. You will never get the carbs set up properly if you are
In reply to a message from Craigy sent Tue 21 Jun 2005:
Hello Craig,
Yes, the air valve could be stuck open, but you have to dismantle
it to see, it could also be the diaphragm. Has it ever worked
properly since it was ‘‘refurbshed’’. After the car has been running
and switched off, you should be able to hear the air valve open and
shut several times when you step on and off the brake pedal until
the vacuum resevoir is full (empty). Does the car roll freely off
the brake ? I would first block off the vacuum line to the rear of
the intake manifold, reset the carbs and test the brake performance
without the servo. I doubt you are getting any boost with a
constant leak at the filter. Then test the servo with a vacuum
pump. If there is a constant leak, start by dismantling the air
valve.
Paul–
The original message included these comments:
OH GREAT!!
Do you think it may be the valve that’s stuck or the servo itself.
I’m sure that the servo was refurbshed about 3 years ago (in fact
looking at my parts catalogue, servo repair kits were bought by the
previous owner for both the fluid & Vacuum sides.
What do I need to strip down to check?
Thanks in advance
Craig
In reply to a message from PS sent Tue 21 Jun 2005:
Thanks Paul you are a great help.
Now try this one for size.
Last night I connected a long pipe to the Vent pipe and ran it into
the car whereby, without starting the engine (kids asleep generally
in trouble with wife already), I both sucked & blowed into the vent
pipe.
The first time I did this there was little resistance, however as
soon as I pressed the brake pedal it appeared to pulsate and work
as a valve. If I blow with the brake off it is air tight. As I put
the brakes on it lets some air through and then becomes tough
again. If I suck with the brake on then some air is let through
momentarily, and if I suck with the brake off then it becomes tough
again.
In reply to a message from Craigy sent Wed 22 Jun 2005:
Craig,
I, for one, think that if you keep sucking on the vent tube
like that, you’ll eventually end up with a mouth full of
brake fluid
Other than that, it sounds like the reservoir valve is doing
it’s job.
I think you mentioned that the servo had been rebuilt a few
years ago. If it wasn’t done professionally, that might be
the source of your vacuum leak. I’ve never done it myself,
but I’ve heard that rebuilding those things can be tricky.
Jacques–
The original message included these comments:
The first time I did this there was little resistance, however as
soon as I pressed the brake pedal it appeared to pulsate and work
as a valve. If I blow with the brake off it is air tight. As I put
the brakes on it lets some air through and then becomes tough
again. If I suck with the brake on then some air is let through
momentarily, and if I suck with the brake off then it becomes tough
again.
What do you think guys?
Gents, I have a question along the same wavelength: my vacuum reservoir
tank is rusted out and the check valve is cracked. Can my brakes still
work (when engine is running, of course) if I were to withdraw the
broken check valve, bypass the reservoir tank altogether and run the
hose straight to the servo booster? I’m not trying to be cheap but
rather buy time until I can deal with those items, if they aren’t
absolutely vital for safety and basic brake operation. I know the tank
is to help you stop the car when the engine is not running, especially
since the Mark X/420G is one big mutha. Without the booster, I can
imagine the driver would have to stand up on the pedal to bring her to a
halt. No, I haven’t tried that yet…
Lenny B. Ramjattan, N4WZQ @Lenny_B_Ramjattan_MC
1966 Jaguar Mk X, 1D76577BW
Pembroke Pines, Florida, U. S. A.
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In reply to a message from Lenny B. Ramjattan sent Fri 24 Jun 2005:
Lenny,
I don’t think so, but the brakes will still work if you cap
off the vacuum line at the manifold.
Granted, at first, it will feel like you’re trying to stop a
fully loaded 1949 Peterbuilt, but if you’ve got the leg
muscle, the car’s still got the brakes.
I’ve been driving without power assist for a while now, and
I’ve gotten so used to it that I’ve put rebuilding the servo
at the bottom of my priorities list.
Jacques–
The original message included these comments:
Gents, I have a question along the same wavelength: my vacuum reservoir
tank is rusted out and the check valve is cracked. Can my brakes still
work (when engine is running, of course) if I were to withdraw the
broken check valve, bypass the reservoir tank altogether and run the
hose straight to the servo booster? I’m not trying to be cheap but
rather buy time until I can deal with those items, if they aren’t
absolutely vital for safety and basic brake operation. I know the tank
is to help you stop the car when the engine is not running, especially
since the Mark X/420G is one big mutha. Without the booster, I can
imagine the driver would have to stand up on the pedal to bring her to a
halt. No, I haven’t tried that yet…
Yes, but the amount of assistance you get from the booster will vary with
your manifold vacuum directly and instantaneously.
Cheers,
Jack Verschuur.
Gents, I have a question along the same wavelength: my vacuum reservoir
tank is rusted out and the check valve is cracked. Can my brakes still
work (when engine is running, of course) if I were to withdraw the
broken check valve, bypass the reservoir tank altogether and run the
hose straight to the servo booster? I’m not trying to be cheap but
rather buy time until I can deal with those items, if they aren’t
absolutely vital for safety and basic brake operation. I know the tank
is to help you stop the car when the engine is not running, especially
since the Mark X/420G is one big mutha. Without the booster, I can
imagine the driver would have to stand up on the pedal to bring her to a
halt. No, I haven’t tried that yet…
Lenny B. Ramjattan, N4WZQ LBR700527@netzero.net
1966 Jaguar Mk X, 1D76577BW
Pembroke Pines, Florida, U. S. A.
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Early disk brake equipped cars did not have a reservoir but rather had the
booster connected directly to the vacuum source as you envision. Usually,
when you put on the brakes you are not also at full throttle so there is
plenty of vacuum available.
Gents, I have a question along the same wavelength: my vacuum reservoir
tank is rusted out and the check valve is cracked. Can my brakes still
work (when engine is running, of course) if I were to withdraw the
broken check valve, bypass the reservoir tank altogether and run the
hose straight to the servo booster?
Lenny B. Ramjattan, N4WZQ
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I have run the line directly from the manifold to the booster without
problems. It is my understanding the reservoir was to provide one last
assist should the engine die. at least one of the restoration books
indicate they simply bypass the reservoir.
george leicht
At 10:58 PM 6/23/2005 -0400, Lenny B. Ramjattan wrote:
Gents, I have a question along the same wavelength: my vacuum reservoir
tank is rusted out and the check valve is cracked. Can my brakes still
work (when engine is running, of course) if I were to withdraw the
broken check valve, bypass the reservoir tank altogether and run the
hose straight to the servo booster?
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True, you can run the servo directly from the manifold vac, but the tank is
not only providing ‘reserve’ vacuum. The valve in the system results in the
vac tank holding max vacuum, thus ensuring the same amount of assist at all
times.
When braking you are most likely to have your foot off the accellerator,
thus having a lot of vac anyway, but the amount of assist will vary with
RPM.
Cheers,
Jack Verschuur.
Hello Lenny:
I have run the line directly from the manifold to the booster without
problems. It is my understanding the reservoir was to provide one last
assist should the engine die. at least one of the restoration books
indicate they simply bypass the reservoir.
george leicht
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