[xj40] Vacuum brake booster conversion

XJ40
1

OK, Here’s a good one…

I’ve finally started to seriously think about converting my '89
XJ40 to vacuum assisted brakes…AGAIN! The leaking mineral oil is
getting old. The only thing that’s kept me from doing this already
is the concern I have over the addition brake pedal pressure that
I’m afraid this will result in.

I purchased a rebuilt brake booster several years ago (for a 1986
XJ6) and even went so far as fabricating the adapter plate out of
aluminum. I also love the thought of getting rid of all the
addition hardware associated with the hydraulic brake system.

Here’s my question…Is the pedal pressure directly related to the
design (or size) of the booster or does the amount of vacuum
supplied to the booster also play apart. Can any of this be
adjusted? I like a lot of braking with a light pedal force.

Anyone with any experience in this area? I guess what I’m looking
for is a real ‘‘Airhead’’ or more to the pont a true ‘‘Vacuumhead’’!–
Groove, 1989 XJ6 3.6L
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2

In reply to a message from Grooveman sent Wed 14 May 2014:

I have questions about this conversion as well.
Specifically- what vacuum boosters will work? I have access
to an older than my 1988 xj6 booster from a friend who is
liquidating his junkyard. He has given me carte blanche on
any Jaguar parts I want on his two remaining cars. From the
newer Vanden Plas I’m going for the bulb failure modules and
I am going to pull the brake booster from this year as yet
undetermined xj6 as well- other than this the cars are
picked over.

But I’m betting that there is some American booster out
there that will work with an easily made adaptor plate and
if anyone knows what this part is I would be appreciative.
The well advertised, and expensive conversion kit does not
include the booster nor does it tell which one is needed
prior to purchase.–
Mogrits
South Carolina, United States
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3

In reply to a message from Grooveman sent Wed 14 May 2014:

You rang? I’m lurking from the XJ forum, but according to
my wife I’m a super vacuum head, so I will respond. :slight_smile:

The boost is proportional to the diameter of the booster
squared (cross-sectional area) AND to the amount of vacuum
supplied by the manifold (stored in a canister via a one-way
valve). The product of the two determines the boost. A
couple of footnotes: you can use a vacuum pump to supply
more than manifold vacuum (up to about 28 inches Hg;
manifold vacuum is around 20), and you can fit a double
diaphragm booster that approximately doubles the boost for a
given cross-sectional area. All IMHO.–
The original message included these comments:

Here’s my question…Is the pedal pressure directly related to the
design (or size) of the booster or does the amount of vacuum
supplied to the booster also play apart. Can any of this be
adjusted? I like a lot of braking with a light pedal force.
Anyone with any experience in this area? I guess what I’m looking
for is a real ‘‘Airhead’’ or more to the pont a true ‘‘Vacuumhead’’!

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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4

That’s a tough act to follow! Very informative.

I did the conversion maybe 10 years ago, using the the kit from the SF Bay
area.

Groove, it’s been so long, I can’t give a very specific reply about pedal
effort. I recall that the brakes did feel different. The pedal effort is
“normal”. Not touchy, but certainly not needing any real force on the
pedal.

I’ve been very happy with the conversion.

Wish I could be more helpful.

Cheers, Alan

In reply to a message from Grooveman sent Wed 14 May 2014:

You rang? I’m lurking from the XJ forum, but according to
my wife I’m a super vacuum head, so I will respond. :slight_smile:

The boost is proportional to the diameter of the booster
squared (cross-sectional area) AND to the amount of vacuum
supplied by the manifold (stored in a canister via a one-way
valve). The product of the two determines the boost. A
couple of footnotes: you can use a vacuum pump to supply
more than manifold vacuum (up to about 28 inches Hg;
manifold vacuum is around 20), and you can fit a double
diaphragm booster that approximately doubles the boost for a
given cross-sectional area. All IMHO.

The original message included these comments:

Here’s my question…Is the pedal pressure directly related to the
design (or size) of the booster or does the amount of vacuum
supplied to the booster also play apart. Can any of this be
adjusted? I like a lot of braking with a light pedal force.
Anyone with any experience in this area? I guess what I’m looking
for is a real%

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5

In reply to a message from agrossman@pacific.net sent Thu 15 May 2014:

Groove

Did this myself a few months ago - very pleased with result

The braking appears a little more sensitive at light
braking but does need a little more foot power when heavy
braking needed.

The space in the engine room is amazing now that everything
to do with the ‘old’ system has gone - can now actually see
the steering motor and the ac compressor, also the front of
the engine in way of the CPS is now easy to get to

The main concerns I had were the following _ drilling and
tapping the inlet manifold for the vacuum takeoff - there
was a ‘blank’ near to the vacuum takeoff for the vacuum
tank driving the ac solenoids so just removed the inlet
manifold ‘loop’ in that area and stuffed a rag in there to
catch the drilling and tapping waste so it did not get into
the inlet itself - turned out to be a lot easier than I
thought it would be.
Some have mentioned that the existing takeoffs for the
vacuum that are already on the inlet manifold would be
enough to drive the booster - personally don’t think it
would do the job and I believe you need a dedicated takeoff
with larger diameter hose - when you compare the size of
the booster itself, the size of the hose connection on the
booster and the size of the existing takeoffs then you will
see what I mean

The other point is that before the master cylinder can be
mounted to the vacuum booster you have to drill the holes
out a bit as the booster bolts are bigger then the
hydraulic system bolts.

There is a good write up on what you have to do in the XJ40
book - follow that and you wont go far wrong

Im very pleased with the result, car just went through the
government annual test here last month and the testers
never noticed, brake response on the test rig was excellent

All I can say is do it and not worry about the green oil
any more, let alone the gas bottle !! - the blank for the
front engine casing where the hydraulic pump outlet is
mounted is available as a jag part - sorry cannot remember
the number but it cost a peanut or two

Best luck and if you want any other guidance just email me
direct

Nigel–
1987 XJ40 3.6 Auto Sov SAJJHALH3AA512874
Singapore, Singapore
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6

In reply to a message from Nigel Snowden sent Thu 15 May 2014:

Nigel
Do you recall the donor vehicle for the booster?–
Mogrits
South Carolina, United States
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7

In reply to a message from Grooveman sent Wed 14 May 2014:

Groove,

I have never, ever, ever, regretted doing the brake
conversion…ever!!

10 years ago now, there was a slight difference in the feel
of the pedal, but over a few weeks, it was unnoticeable.

I’ll try to find the old paperwork I had for acquiring the
parts and what was specified for the vacuum in relation to
the plate I purchased.–
The original message included these comments:

I’ve finally started to seriously think about converting my '89
XJ40 to vacuum assisted brakes…AGAIN! The leaking mineral oil is

–
Rob Evenson, '89 XJ40/XJ6, VIN 583143, 125K
Mobile, AL, United States
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8

In reply to a message from Grooveman sent Wed 14 May 2014:

Dennis. In case you decided to proceed I have emailed you all the
instructions including what booster to use.You will receive a
picture of the booster adaptor plate and cover plate for the pump
which you will remove.These are actual size from scans. John–
John. Walsh. Purrrrr 89 vdp
bowmanville ontario, Canada
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9

In reply to a message from h s law sent Thu 15 May 2014:

So, the proper booster is a State Secret? No one seems
willing to share this basic information on the forum.–
The original message included these comments:

Dennis. In case you decided to proceed I have emailed you all the
instructions including what booster to use.You will receive a
picture of the booster adaptor plate and cover plate for the pump
which you will remove.These are actual size from scans. John

–
Mogrits
South Carolina, United States
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10

In reply to a message from Mogrits sent Thu 15 May 2014:

Mogrits…

I decided to do some research on the net in respect to vacuum brake
boosters and as Werner Klemperer (aka Col. Klink) would say, the
results were ‘‘Veryy interesting’’

There are all kind of formulas that take the size of the Booster
(actually the size of the rubber diaphragm)and the amount of vacuum
applied to come up with the actual pressure excreted on the master
brake cylinder. There are also single and double diaphragm
boosters just to muddy the water.

I believe the devil really lies in the placement of the studs on
the booster and if they will line up with the master brake cylinder
and the brake bracket on the opposite side.–
Groove, 1989 XJ6 3.6L
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11

In reply to a message from Grooveman sent Thu 15 May 2014:

I’m going to pull the booster from the older xj6 at my
friends junkyard/breaker and see if it will fit. If I can
find the VIN on the old car I will post it so people will
know what can work. The XJ40 Book references using a 1982
vacuum booster but ‘‘the kit’’ just says ‘‘commonly available
booster’’ at estimated 150 dollar cost. Well, the kit itself
is pricey for one adaptor plate and one blanking plate and
the booster may be only 150.00 but the core charge is equal
to that, from what I’ve found.–
Mogrits
South Carolina, United States
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12

In reply to a message from Grooveman sent Thu 15 May 2014:

Sorry…

My spell check program changed ‘‘Excerted’’ to ‘‘Excreted’’. Which
would then mean the amount of pressure the booster ‘‘dumps’’ on the
master brake cylinder!!–
Groove, 1989 XJ6 3.6L
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13

In reply to a message from Grooveman sent Thu 15 May 2014:

Groove,

I personally enjoy ‘‘pressure excretion!’’ Got a good laugh
out of that!

I think what you were looking for is spelled ‘‘exerted’’
with no ‘‘c.’’ :slight_smile:

Cheers,

Don–
The original message included these comments:

My spell check program changed ‘‘Excerted’’ to ‘‘Excreted’’. Which
would then mean the amount of pressure the booster ‘‘dumps’’ on the
master brake cylinder!!

–
Don B : '04 XJR '93 XJ40 VDP ex-'88 XJ40 Sov
Franklin, TN, United States
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14

In reply to a message from Mogrits sent Thu 15 May 2014:

My local DIY salvage yard would charge $15 for a vacuum
booster with a $2 core. And $10 for a brake master
cylinder, $2 core. Regardless of make/model. I got my
vacuum booster from an 83 XJ6 at that yard, but the booster
would be the same for an XJ6 all the way back to 1969.
I don’t think you need to be specifically Jaguar for donor
brake parts. As long as the brake pipes match, you should
be good to go with one for a Buick even.–
The original message included these comments:

know what can work. The XJ40 Book references using a 1982
vacuum booster but ‘‘the kit’’ just says ‘‘commonly available
booster’’ at estimated 150 dollar cost. Well, the kit itself
is pricey for one adaptor plate and one blanking plate and
the booster may be only 150.00 but the core charge is equal
to that, from what I’ve found.

–
Pete Peterson 70E(193K) XJ40s(88-270K,89-97K, 94-122K)
Severna Park, Maryland, United States
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15

In reply to a message from Jaguarpete sent Thu 15 May 2014:

Groove

Donor car was a 1985/6 XJ6

Nigel–
1987 XJ40 3.6 Auto Sov SAJJHALH3AA512874
Singapore, Singapore
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16

In reply to a message from Nigel Snowden sent Fri 16 May 2014:

Thank you to Nigel and JaguarPete for stating which boosters
will work.–
Mogrits
South Carolina, United States
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17

In reply to a message from Mogrits sent Sun 18 May 2014:

There’s a guy in Portland, Oregon named ‘‘Booster’’ Dewey who
sold me a vacuum booster (with a core charge) some time ago.
I had good luck with it on my 76 XJ6L. You may want to
Google him. But a vacuum booster from your local DIY
salvage yard should work just as well.–
The original message included these comments:

Thank you to Nigel and JaguarPete for stating which boosters
will work.

–
Pete Peterson 70E(193K) XJ40s(88-270K,89-97K, 94-122K)
Severna Park, Maryland, United States
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18

In reply to a message from Jaguarpete sent Sun 18 May 2014:

I did the conversion on my '88 xj6 years ago and didn’t
notice much difference in braking effort. I would say it
was less than the difference you feel after a good brake
bleeding job. The booster I used was for an '87 xj6 if I
remember correctly. I bought it rebuilt with a lifetime
warranty for around $70 at a major chain auto parts store.

Its a great update IMHO.–
1994 XJ12 - Barn find, previous 1988 XJ6
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19

In reply to a message from Jaguarpete sent Sun 18 May 2014:

I did the conversion on my '88 xj6 years ago and didn’t
notice much difference in braking effort. I would say it
was less than the difference you feel after a good brake
bleeding job. The booster I used was for an '87 xj6 if I
remember correctly. I bought it rebuilt with a lifetime
warranty for around $70 at a major chain auto parts store.

Its a great update IMHO.–
1994 XJ12 - Barn find, previous 1988 XJ6
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20

In reply to a message from DolphS sent Mon 19 May 2014:

Guys…

As I mentioned previously I’ve already purchased a remanufactured
brake booster for a 1986 XJ6 (I understand this is the one normally
used for this conversion) but I don’t think I’m going to use it.
As far as I can tell it’s a single diaphragm booster (how do you
know?) and I think I’ll need less pedal pressure with a dual
diaphragm booster of roughly the same size. Here’s the one I’m
looking at:

https://www.performanceonline.com/8-DUAL-DIAPHRAGM-POWER-BOOSTER-
ZINC-SRB-8D-SRB-8D/–
Groove, 1989 XJ6 3.6L
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