[xj40] Vacuum brake booster conversion on my 1989 XJ40

Gentleman…

I just received my ‘‘dual diaphragm’’ 8’’ vacuum brake booster and
there�s a lot I like about it; Considerably smaller than the 1986
XJ6 booster that is normally used. It has a tapered profile
toward the master brake cylinder so it should clear the radiator
header tank assembly with ease. And it has a very attractive
semi-gloss powder coating finish. The dual diaphragm should give
me superior braking performance with the same amount of pedal
travel. The company said it generates more force than an 11 or 12’’
single diaphragm booster…We shall see.

The only downside is that the bolts that mount the master brake
cylinder are 1/8’’ too close together. I’ll have to Dremel in the
two holes in the master cylinder 1/16’’ each. A minor annoyance.

So now I’m fabricating the adapter plate and have a question…

It seems that to keep the master brake cylinder level (the mounting
holes on it appear to be about 45 deg off level) that the booster
bolts going through the adapter plate will end up rotated to the
12, 3, 6, 9 o’clock position. How do you secure the bolt that’s in
the 6 o’clock position? With the adapter plate in place there’s
only about 3/8’’ between the plate and the body of the pedal
housing.

What say Ye all you ‘‘conversion boys’’–
Groove, 1989 XJ6 3.6L
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In reply to a message from Grooveman sent Sat 14 Jun 2014:

Hi Dennis.If I sent you all of the kit instructions previously the
info you need is on page 2 para 10 and 11. Does the booster you
got fit the adaptor plate you made if so the 4 rear mounting studs
for the booster will be oriented at 1 oclock 4 oclock 7 oclock and
10 oclock. I dont know if thats AM or PM pm.!!! John–
John. Walsh. Purrrrr 89 vdp
bowmanville ontario, Canada
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In reply to a message from h s law sent Mon 16 Jun 2014:

Dennis

I gave up trying to get the bottom nut on and just made
sure the other 3 were tight !!

Nigel–
1987 XJ40 3.6 Auto Sov SAJJHALH3AA512874
Singapore, Singapore
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In reply to a message from Grooveman sent Sat 14 Jun 2014:

Thanks John and Nigel for the reply…

In my post I mentioned that the mounting holes on the back of the
master brake cylinder are canted 45deg., but that was just an
eyeball guess. I made a small jig out of popsicle sticks that
allowed me to use a small inclinometer and the actual orientation
is 30deg. from level.

So to mount the brake booster the top of it has to be rotated 30deg
toward the engine (I have a left hand drive car). This will result
in the booster to adapter plate bolts being at roughly the 1,4,7
and 10 O�clock position. But because of the width of the pedal box
the 1 and 7 O�clock bolts still don�t clear it. The only solution
I can see is to cut those two bolts so they only extend beyond the
adapter plate by the width of the bolt. Then slide the nut between
the adapter plate and pedal housing and thread the booster on those
two bolts first�.Oh well�As my wife always tells me when I complain
about a car problem��You�ll figure it out�!

Next question, I see where some sites talk about removing the pedal
box to do this conversion? Is this really necessary ? Can�t I
just do everything through the two round inspection holes on the
side of the pedal box?

The adapter plate I made for the �86 XJ6 booster won�t fit the bolt
pattern on my new Uber-booster so I�ll have to fabricate a new one.

Just out of curiosity does anyone know approximately how much
vacuum the 3.6L engine produces at idle ?–
Groove, 1989 XJ6 3.6L
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In reply to a message from Grooveman sent Tue 17 Jun 2014:

Dennis. To be installed properly the 1 oclock and 7 oclock studs
need to be trimmed 1/2 inch. Once the upper and lower studs have
been shortened verify that you can easily thread the slim
mounting nuts on to them. Remember that the vacume fitting must be
down. The trick is SLIM NUTS. John–
John. Walsh. Purrrrr 89 vdp
bowmanville ontario, Canada
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In reply to a message from h s law sent Tue 17 Jun 2014:

John…

When you say the studs need to be trimed 1/2’’ I take it you mean
1/2’’ beyound the adapter plate. That’s the only way I can see to
make it work. Obviously you know EXACTLY what I’m talking about !!–
Groove, 1989 XJ6 3.6L
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In reply to a message from Grooveman sent Tue 17 Jun 2014:

Dennis we are talking about the same thing, you did not confirm
that I sent you the written instructions and scans if you never
received them I can email them tomorrow at noon.John–
John. Walsh. Purrrrr 89 vdp
bowmanville ontario, Canada
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In reply to a message from Grooveman sent Tue 17 Jun 2014:

Groove

No need remove the pedal box - work through the small holes
either side - getting the clip off the pivot bolt is a pain
but it will come with some jiggling !
Getting it back on is an adventure as well - fortunately if
you drop it it ends up on the footwell carpet (that
happened a few times when I did mine !!)

Nigel–
1987 XJ40 3.6 Auto Sov SAJJHALH3AA512874
Singapore, Singapore
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In reply to a message from Nigel Snowden sent Wed 18 Jun 2014:

Nigel and John…

Thanks Nigel for the info. It didn’t make sense that I would have
to remove the pedal box to hook up the linkage, but you never know.

Thanks also John, I did finally track down the installation
instructions you sent me.

I finished fabricating my new adapter plate yesterday so hopefully
I’ll get this project done in the next day or two.–
Groove, 1989 XJ6 3.6L
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In reply to a message from Nigel Snowden sent Wed 18 Jun 2014:

I replace my clevis pin and retaining clip with a bolt and a
Nylock nut. There’s just enough room.–
The original message included these comments:

Getting it back on is an adventure as well - fortunately if
you drop it it ends up on the footwell carpet (that
happened a few times when I did mine !!)


Pete Peterson 70E(193K) XJ40s(88-270K,89-97K, 94-122K)
Severna Park, Maryland, United States
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Good evening everyone. I’m also about to start this adventure to replace the hydraulic booster with a vacuum one. Since I can’t find the booster described in this post I would like to use another one. My question is how long should the rod be from the booster to the brake pedal?

Wow … interesting to see the thread I started 10 years ago.

First thing first … The smaller booster I installed DID NOT WORK !!

Although it actually functioned it did not provide enough brake pressure to lock the brakes in a panic stop. After all the trouble of modifying the mounting for the new booster I could tell right away it wasn’t up to the task. So I ended up using the go-to booster for this modification … one from a series III XJ-6.

I believe I addressed this disappointment in later posts.

The XJS brake booster from 1975 - 1988 will also work, probably the same as the series 3 xj6.
Jaguar sourced the booster from the GM parts bin from the late 60s through the early eighties models. I used to have the list of which ones they were, but if you take a close look at them, you’ll be able to figure it out. I got a new one from Jag-spares in the UK it was about $225 including shipping to the US

I finally found a series 1 booster at moss motors,$247, I had to modify the mounting holes but I have brakes again.

David …

Good news on sourcing the booster, So a series I booster worked. How about a picture of the install and how do the brakes feel ?

Screenshot 2024-07-02 at 7.07.13 PM

Why do you want to remove the hydraulic brake force multiplier, it is far more powerful than the vacuum one.

Had to redrill the mounting holes but otherwise straight swap. sadly it has sat too long, starts and idles fine but no power, won’t even move in the grass. the plugs are dry and carbon fouled so I will start there.