There is a workshop manual for the Series 3 XJs. Its about 1.5" thick, with a dark green cover, and I used to have one when I had my 1980 XJ. I would strongly recommend you try to find one; maybe on eBay ?

As I recall, the brake servo (or booster as it is called in the US), bolts onto the aluminium pedal box, which itself is bolted onto a platform forming part of the bulkhead. The pedal linkage goes into the back of the servo, and the master cylinder bolts on the front of it. It is probably better to first remove the master cylinder, then to take off the whole pedal box/brake levers/servo etc as one assembly.
I dont think there is anything repairable in the servo, as special tools are necessary to assemble it, (I think !!).

The pedal box comes out complete with pedal levers. You have to take the pedal off to allow the lever to come out through the bulkhead hole

 

Fraser has it right, in a nutshell.

But it CAN be removed with the pedal box in place. I seriously doubt that doing so results in any savings of time or effort. Ask me how I know :-)

Cheers
DD

 

New service manual:

http://www.motorcarsltd.com/AKM9006.html

 

That's the manual I had. (from ken@britishparts.com)

Well worth splashing out on if you are keeping the car. It is an absolute MINE of information, as it was written by blokes who knew what they were talking about. Sadly they are now all retired or dead !!

 

I did this job. I disconnected the brake lines from the master cylinder and removed it along with the booster and pedal box. I can't remember if the master had to come off the booster to get everything to come out, but I think so, and it is only two nuts to get it off the booster. Then there is room to wiggle the booster/pedal box out. The pedal box has bolts that go in from under the dash, One of them is a different size than the others, and some extensions are needed to get the socket onto the bolts. There is a wire connection to disconnect on the brake pedal for the brake light switch. No need to remove the pedal, it will fit through the hole at the right angle when lifting it out. Taking out the pedal box allows easy access of all four nuts that hold the booster to the box.

With everything out, you can clean up damage from leaking brake fluid and repaint if you want. Also, you will need to seal up around the edges of the pedal box when you reinstall it. I used silicone and it seems to be fine.

The brake fluid leak on mine was from the brake fluid reservoir seal rings where it attaches to the master cylinder. I replaced the reservoir because the bottom was warped badly, and I used some Viton O-rings from a pack of various sizes. Bought the o-rings from the HELP section of Autozone, they are the brown colored o-rings.

I think a different type of master cylinder leak can result in a bad booster. If the master leaks into the booster, keep in mind it might require replacing of the master so the replacement booster doesn't fail.

 

Here is a description from a post on jag-lovers, how to change the booster without pulling the master cylinder and pedal box out:

On 9/2/07, Bruce Hugo <brucehugo at hotmail.com> wrote:
> My brake booster is 'sucking wind' so to speak and my right leg is
> getting a good workout, but at my age stopping nearly 5,000 pounds
> without brake assist is no longer an option. The ROM says the way
> to replace/remove the booster is to pull everything - pedal, pedal
> housing and booster which seems to be a bit of overkill. Anyone
> succesfully performed this task? Suggestions?

NO, if it is a series 3, you don't have to remove it all. I did the
job in a hotell parking lot in Washington DC with only my "traveling"
tool kit. I wrote it up in the book "The XJ6 Jaguar from Bumper to
Bumper" which you can read on this site. But here is a copy of what I
wrote::

The Haynes manual says that you have to remove the entire assembly
with the master cylinder and the pedal box assembly all in one piece.
This requires that the brake system be opened with its attendant
bleeding after reassembly. It is also a bitch of a job.

When I read the manual and realized I would have to bleed the brakes
by myself I decided to find out if there was a better way. There is,
at least on a series 3 with LHD. In looking at other models you may
want to make sure that the master cylinder will move out of the way
and that you can get to the clevis pin.

First remove the two bolts that fasten the master cylinder to the
booster. Then remove the vacuum hose from the booster. Now comes the
ticklish part. There is a rubber plug on the side of the pedal box
that when removed gives access to the booster clevis pin. There is a
rubber plug on both sides, remove them both. Now with a pair of needle
nose pliers or a screw driver or anything you can get into the hole,
remove the cotter pin (split pin) from the end of the clevis pin. Be
very careful here since anything you drop will not be recoverable.
Don't drop the pin, or the washers or the clevis pin. Withdraw the
clevis pin and the booster is now disconnected from the brake pedal.

Now remove the nuts from the four studs that fasten the booster to the
pedal box. Again, be careful here as things you drop may never be
found again. Once these are all removed the booster can be pulled
forward. It takes a little jiggling and twisting, but it will snake
off the studs. The clamps that hold the brake lines to the inside of
the fender (wing) well will have to be removed to give you enough
slack to move the master cylinder. You will have to slightly,
CAREFULLY, GENTLY bend the brake lines to move the master cylinder out
of the way. Be very careful here as you don't want to crimp a brake
line.

Putting the thing back in is a little tricky as it will only snake
back in in one way. Its like those little wire puzzles that when you
find the trick are easy, but are impossible without knowing the trick.
Just wiggle it around till it goes on. It can help to get one nut on a
stud then wiggle the thing around until the remaining studs go in. A
slight amount of prying with a big screw driver can help, but you
should be very cautious with this it can result in breaking something.

Once it is back in position get the nuts on all the studs but don't
tighten them yet. Now re-attach the master cylinder and the clevis to
the brake pedal. Again, I warn you, for Gods sake don't drop anything
while putting the clevis pin back in or you will have to remove the
pedal box to retrieve it. When the clevis is reinstalled with a new
cotter pin (split pin) you can tighten everything back up and
reinstall the vacuum line and the rubber plugs and top up the brake
fluid resevoir.

The above is a direct quote from my book. You need to download it
anyway, but this will tell you why.

Jim Isbell