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Brake Servo at fault?

17079 Views 41 Replies 6 Participants Last post by  buddyarrows
So I have just replaced the steel rear brake pipes from the unions over the back axle to the flexible pipes because they were corroded and failed MOT. Used copper replacement piping. All went smoothly. So it seemed.

Starting the car up, I took it for a little test in our street. Nice brakes on first press of the pedal, then on immediate second press the pedal comes back up at me and it feels like the servo is not working. If I leave it a few moments then brake again its okay. Brake immediately again and the pedal forces itself back up till its solid. After that no servo till I give it a few moments again to recover.

So I call up a friend who is a mechanic and he thinks it might be a vacuum leak. He checks the hoses and check valve which breaks in his hand. No worries, he says, and araldites it together. After an overnight curing the car is still doing the same thing. So I replaced the check valve and hoses. Still got the same problem.

Called up the dealer/service agent who said that it sounds like the servo could have an internal leak. They are way too busy for the next two weeks to look at it, he told me (brilliant for them!), and added that a new unit from Jaguar will cost "hundreds of pounds". It is reasonable, he added, to source a part from a breaker as these things rarely fail. So I went online and began getting quotes...

Before I start taking my car apart (with all my inexperience in these things) I would really like to get a second expert (hopefully) opinion on all this. I wonder, for example, how this happens exactly at the time I replace pipes at the other end of the car? Is there a connection?

Thanks in advance for any input!

(X-Type turbo diesel 2l manual)
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ensure that you get the wurz number off your existing Servo on the white label generally, as you should get another servo with the same number I got mine from XJ Engineering Ltd, Surrey 01420 23001 £140 plus vat
WURZ number

Thanks Barney - I shall check that out...
Couple of thoughts. Did you bleed the hydraulics thoroughly after changing the pipework (I only ask as you haven't said you did)

Secondly, the simplest test for the servo is, with the engine switched off, pump the brakes a number of times to dispel all vacuum from the system. Then, with footbrake pressed hard down, restart the engine. A good servo should noticeably pull the pedal down as you maintain your push on it
Hey Astromorg. Thanks for that. We bled the system several times thinking that might sort it out but with no luck. Even the front set which were never touched.

My mechanic friend suggested the servo test you describe. The result was no difference in feel as the engine started. Still rock solid. Boost effect only comes in after the engine runs with no pressure on the pedal. Then disappears after braking once or twice in succession or holding the brake pedal down.

So I guess I better order the servo replacement. I got quotes on several online from breakers. Need to check the WURZ numbers as advised by Barney. Looks like it will cost me around £60 for a second hand one with a year warranty.

So...any other reason why I might be making a mistake here?

Thanks in advance for all your inputs.
Hey Barney. Just had a look for that WURZ number...found a Bosch sticker with two barcode numbers and another number like this:

The first number is the biggest. Which one is the WURZ number? (feeling stupid here)

And by the way, don't know if its significant but when looking for this number I noticed that all the paint on the front of the servo below the master cylinder has peeled off and there is a little rust on the servo case and it seems slightly damp from what I guess is brake it just spill from over the years or do I have a leak?
The first number is the wurz number, I don't know about the dampness it may be a leak from the master cylinder, but the servo should not be rusty
Thanks Barney.
I was just about to order a servo from a breaker when someone suggested it could also be a fault in the vacuum pump. Is this so and how would I test for that before ordering a servo when I may well not need one?
you could check it with a vacuum gauge I don't know what you should get but I expect somewhere around 20 inches should be okay, A simple way to test the system is to pump the brakes five or six times with the engine off to discharge the accumulator. Then press down hard on the pedal (about 40 lbs. of force) and start the engine. You should feel the pedal fall slightly when the engine starts, then rise.
Thanks again Barney. As mentioned above in response to Astromorg, tried the pump-and-start test with negative results - no discernible change in pedal feel. Are you saying that this rules out the vacuum pump being at fault?

If I remove the pipe that goes from the vacuum pump to the check valve, start the engine and place my thumb over the hole where the pipe came out of the pump, how strong should the suction feel? Or is it dangerous to do that if the pump is working correctly? I don't have a gauge...
I'm dealing with your exact problem myself!!! .. I got a replacement servo from the other day with the master cylinder attactched for £50 ... All I done was changed the the back and front discs and pads!!

My car has just dumped a trouble code (P0100) MAss Air Flow fault and my EGR warning on the reader is flashing indicating that it's def a vaccum issue!!

Like you I checked all the hoses too, eve took the check valve off and it's fine! :-/
Jamber - you have a reader??!! You are way more hi tech than me about this. I have completely rebuilt engines (including two 4.2 jag motors) in my time but not since the 80's. The only vacuum issues one had then were on the auto advance on the distributor or the servo diaphragm which you could replace quite cheaply. Or of course a manifold gasket.

I'd be interested to know what a mass airflow fault is (connected with the turbo I suppose - which is actuated by vacuum amongst other things I guess) and what an EGR warning is. Was the new servo tested and guaranteed by the supplier?

If I don't get some input tonight about how I can check the vacuum pump without a gauge I am going to try my idea as above with my thumb in the morning...

I will keep posting till I am sorted. Never know - might help someone else...
Hi Ozyris,

You can buy readers fairly cheaply these days! .. My car is doing exactly what yours is doing! I quite happily got in my car, having patted myself on the back for a job well done, then discovered after about 2 seconds of pressing the brakes that the pedal was lifting up against my foot. Like you I automatically thought it was a problem with bleeding the brakes, so I went down to the nearest store and bought a pressure bleeder to be safe.... It made no difference!

I then thought I had damaged the master cylinder by pushing the pistons back in to the cyilinders, forcing the fluid back up the lines. But apparently (having checked the official Jaguar method) I had done it correctly!

I've already checked the check valve and pulled out the vaccum line into the servo, and getting a powerful vaccum from it. so I'm now convinced having done the brake pedal test that it is the servo that's at fault. Well that and the fact two mechanics have also said it's most likely to be that. After all, that's how they check the servo at MOT! If I park the car and leave it for say 30min and go back to it and push the brake pedal before starting it, the pedal is solid, so it def isn't holding vaccum!

As for the fault codes I'm no further forward than you as to why they are showing, I just read it on a post on the net, that a faulty 'booster' can cause this code to show.

I've managed to obtain an official Jaguar workshop cd if you need info on how to remove the servo? I think that's going to be my course of action now :-/

The Servo has a 3 month warranty! :)
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Your thumb will be fine. If the pump is working properly, you should be in no doubt - the'suck' will be strong! (As your thumb disappears down the tube!!!) JTIS gives 12 in Hg as the minimum vacuum (About 6 psi negative) - quite gentle really!!

The reason you can get a MAF fault with the servo problem is that the failed servo can allow an excess of air into the engine at the inlet manifold - more than the MAF sensor measures at its upstream position. Discrepancy = Fault (Usually P0101)
Thanks for the moral support Jamber. Unless anything substantial has changed since those heady days of yore, I seem to remember a servo not being hard to change. Are you doing yours over the weekend? I still haven't ordered my unit yet. If you have any difficulty changing it you can let me know if you would be so kind.

Astromorg - thanks for the fault code and explanation of the MAF fault. I will check my vacuum is healthy before I order the part in the morning. I might drive to Islington and collect it instead of waiting till Tuesday - provided they are open tomorrow...

There are two main great things about this site. The first is the brilliant source of technical expertise. The second is moral support and a sense of cameraderie. We are not alone. Never had that when I rebuilt my 4.2 exec motor hanging from a beam in the garage (the block, not me)...

Very much appreciated!
I'll need to wait till next Thursday to give mine a go.. I had a look at it yesterday and it looks as though it's well bedded in!

Let me know how you get on if you give it a go before Thursday!

Good luck! :)
Well Jamber, I have now spent some time checking the vacuum. I pulled the black hard pipe out of the orange collar in the pump and ran the engine. Then I put my finger over the opening. I have to say that although there is certainly vacuum there I would not class it as strong. Gentle might be a better word. I think if it was stronger the servo would empty quickly enough to function correctly. It only takes about ten seconds before I have proper brakes again after losing them.

I don't know if there are any other lines coming out of the vacuum pump because its hard to see behind there. I think there has to be one going to the turbo but I can't see it. Is there any possibility you could send me a schematic from your manual? If there is slight leak in that line it could, I imagine, cause the symptoms I am seeing. I would like to try blocking that off and then seeing what the vacuum feels like. I think if that makes a substantial difference then it is probably the problem. I think it would be far simpler to sort out a pipe leak than changing the servo. I just remember the local Jag specialist (Chiltern of Bovingdon) saying they have not seen a failure in an x-type servo yet. So I would like to be sure.

Barney, I am not ignoring your expert advice. Once I am sure there is no vacuum leak then I will be following your and Astromorgs wise words. I will then be asking Jamber if he (I assume "he") can share the instructions for the servo work from his CD...

It seems there are only 4 bolts holding it on, so I guess it will be just removing the MC and holding it back out of the way, removing the pipe grommet connector thingy from the servo, unbolting it and lifting it away. The two inboard bolts might be tricky to get to...all those pipes...we shall see...

So, Jamber, could you possibly check on your CD if there is more than one vacuum line coming from your pump and what it connects to? Pretty please? Thank you.

(yours is a diesel too right?)
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Yeah mine is a diesel too ...

I'm at work right now. P.M me your email address and I'll email you the PDF's .. I even noticed last night whilst browsing the cd that it comes with a step by step guide to checking the servo! Everything is suggesting that it is the servo. Now you've got me thinking, especially now that a main dealer is saying it's unheard of!? :-/
Not "unheard of", rather "rare". So its still a major possibility once the easier options have been eliminated.

Address PM'd as requested.
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