If this has happened with out replacement of MC or other components , I would more lean towards the actual cause being the brake booster ...either one of the internal vacuum valves in the booster were damaged with the repeated full stroke pedal pumping or the same action weakened the balance spring ....either one could offset the resting (un-actuated) diaphragm position.....I have had to fix several fords after owners have been a bit vigorous with the brake pedals during a manual bleeding session and knackered the MC.
Also of course a cheapo not to spec Master cylinder could cause the need for adjustment if it was replaced.
I have to admit this has piqued my interest as I've bled hundreds or Mondoes (virtually identical braking system to the X -type) and have never seen this ...I could have just been 'lucky' of course....just wondering I this is a symptom of another fault.
Anyway what ever the cause this is all another very useful tip and trick brought to the fore by dwclapp .... thanks.

 

Yeah, Fords have had this quirk for quite some time. Let us not forget who owned Jag when the X was built, FORD.

 

Sorry it took so long to get back - I just now went back to the car. Before I replaced the MC I tried to shorten the brake rod. I tried using needle nose pliers and a socket to do this. That sucker is solid - isn't moving. So if this rod "extended" somehow it wasn't from the front. Unless something happened on the other side of the booster by perhaps pumping the brakes quite a bit this rod is probably good.

I then replaced the MC. I can tell you that I didn't have to force the new MC into position - it slide right in no problem which tells me if the rod was too long wouldn't the last few mm be pushing against the piston and literally be pushing back on me? That I can tell you didn't happen. The MC went all the way to the bolts with no resistance.

Unfortunately, all brakes are still locking up and the pedal still gets stiff over time thus doesn't take much to stop the car when that happens. I'm out of ideas. It has to the the antilock brake block - what else can it be?

 

I ruled out the booster for the following reason. I disconnected the suction valve to the booster that comes from the engine. That made stopping the car very hard because I no longer had help form the engine. So at this point I figure now it's just my foot that should be pressing on the brakes without any interference. Without the engine helping it took a couple more miles for the same problem to appear but it did still appear. So because of that I ruled out the booster being the problem.

 

I haven't seen any ruptured lines. If this was the case I would have min of two ruptured lines (all brakes locking) which I don't see this happening. The only thing that's left is the ABS block.

 

no no no! go back to the car have some one push the pedal all the way down grab your 7mm socket and get a needle nos vise grip and try again

 

Ok - but if the rod isn't pressing into the MC during install I'm trying to understand the theory here. My assumption is when your foot is off the booster it should always come back to position zero which mean no pressure on the MC piston. If there was no pressure on the MC during installation of the MC and it should always come back I don't see how this is going to help anything. There is some risk messing with this rod adjuster ( I tend to "brake" things when I can't get them to move).

 

Boy, I've encountered some resistant people in my life, but you have to rate in the top 3. Why ask for help and guidance if you just choose to disregard it?

 

Yes Dell - I've read your smart a$$ comments and I choose to ignore you until now. Apparently you're used to people who don't ask questions when asked to jump off a cliff. Sorry but I do have a brain and I do like to use it. Sorry if that offends you. If you cannot answer my concerns about this rod "extension" that you are so passionate about then I'd rather you remain silent. I seem to remember being taught "if you don't have anything nice to say, don't say anything at all". You must have missed that lesson. I stated why I thought this rod was not an issue and instead of you using YOUR brain and explaining it out for me you choose to insult me.

To those who have respectably given me suggestions on this board I truly thank you for your time and expertise. Whatever the cause I find I will eventually post it for future reference.

 

I already ruled out the booster but I am having second thoughts. I did try driving the car without the engine assist value in and I did get the same problems. But what if the booster is still applying pressure on the atmospheric side (manual side) and not releasing as it should? So I have a plan. I will drive the car until the brakes lock again. I will then jack it up, verifying locked wheels. I will then unplug the valve for the engine assist and see if that releases the brakes. It shouldn't based on my previous tests before but I will try as I do have a new MC in the picture now. If that works then I know it's not the ABS - it's the booster. If that DOESN'T work I will unbolt the MC and pull it out far enough to hopefully release the rod. If that works then I know there is something wrong with the booster on the atmospheric side of things. I read that the ABS never applies pressure only takes away but who knows what it does in the event of a malfunction. Why the ABS would all of a sudden be bad doesn't make sense but beating the crap out of the pedal over a few days would make sense why the booster would go bad.

 

of course remember do not discount the Booster even if the releasing the vacuum doesn't free up the brakes , it could be that the balance spring (the internal spring inside the booster) has been weakened.
And even though I've never personally seen this fault on a ford (booster rod suddenly too long without replacing any components) I'd still be listening to voices of experience in the Jag world and would be trying to adjust that rod.
I'm just wondering If you cant adjust the rod could you space the MC away from the booster with a thick gasket ...you'd still need to ensure a very good seal with the booster though.
Its an intriguing and I'm sure very frustrating fault.
Please keep us updated.

P.S
Please don't discount or get upset at Dell ...yes he seems to have a shorter fuse than some ..well online anyway ....but he also seems to be chock full of useful information and is always willing to post links to ideas or past cures...and sometimes its does take a gentle poke from others to get us stubborn types to go back and reconsider a previously discounted cause.... .and remember its all just words on here ...we'd probably all get along just fine in REAL life.

 

 

Actually, I'm more used to people who try researching everything they can on their own before asking for it to be just "given" to them. Hence the search and advanced search option in this forum. More recent generations never had to work for information (oldies, remember being driven to the local library, using the dewey decimal system to find resource, sitting in the library writing your paper). Then when told how to fix something, they actually do it instead of ignoring experienced people who fixed the problem. I admit to not dealing well with dense people, that's my shortcoming.

Being inquisitive is one thing. To fully understand how, why, where something occurs. You're posts show little of this, just disregard for people (many in the links are sort of "gurus" here = Dennis Black, Brutal, etc.) who have years of experience. Have you actually read in FULL the links DW posted above. It TELLS you why it usually happens, including not only rod length problem but possible seal scoring and rod damage.

Yes, I try to excoriate people when they are being obtuse. Take it as an insult or good humor, it doesn't affect my life. I find people like you more of an amusement in human behavior, particularly in cyber space, where nearly everyone personifies a different personality than in real life. That interests me, one of my degrees is in cultural anthropology.

 

Dell - don't bother to post again unless it's to apologize. I spend HOURS if not DAYS researching issues on the internet before I resort to asking for help (I am a software engineer by trade BTW). I am 50 years old - I'm sorry you misjudge me. The rest of your bla bla bla I skimmed and have no desire to comment. I expect an apology and soon...

 

Yes - you are right on target. It was the booster - I will explain below.

As far as Dell perhaps if he got to know me we would be buds but he needs to work on his assumptions and his attitude.

 

I "patched" the problem because I don't consider it a fix. After the brakes locked up again I disconnect the engine pressure value but it did not release the brakes. Only removing the MC from the booster released the brakes. So ok - problem is on the front side of the booster. As greenestate pointed out the problem, in theory, must be related to a weakened internal spring brought about by constant pumping of the brakes during the initial unsuccessful bleeding of the brakes. Makes a lot of sense. There is no sudden "lengthening" of the booster rod - perhaps only a misinterpretation of the problem.

I finally was able to start adjusting the rod length. I initially shortened it all the way to prove or disprove that this would help. After a test drive all was good - no locked brakes but very soft peddle as expected. I then spent several hours "adjusting" the rod, bleeding, drive, repeat, where it would not lock up and still have a nice stiff pedal. Note that I had to disconnect both hydraulic hoses each time to have enough room to remove MC. After the final test drive when everything seemed pretty good I bled all 4 brakes (back brakes left until the end) - pedal responded much better than I though after the 4 brake bleed. Great pedal - no locked brakes except I now feel I could have gone shorter - brakes still feel like they could be "slightly" looser although when all jacked up they do spin freely now but perhaps could be better.

I will keep an eye on it because this is my son's car who wants it at college and I'm afraid if they start locking up again because the "internals" weaken even more he won't know the difference between locked brakes and not locked brakes. That was my primary concern as to "why" this rod magically "lengthened" without an explanation.

Thanks to all who indicated that this could be a factor. It has now started to make sense as to why this ended up being a problem. Purchase a pressure bleeder pump ($50 Amazon) will avoid damaging your booster or MC by pedal pumping.

 

Holy ****, you mean adjusting worked?

 

Lol Dell ... Just LoL .....loving the sarcasm..no really all Brits love sarcasm...we'd proudly call you british...not that being British is all that good at the moment what with the looming Brexit **** up..Anyway at least it's fixed ... Although at least a little bit of me wanted to know whether it's the spring or vacuum valves that 'change' ...but it's fixed so that's the objective I guess.

 

My family is transplanted Brit, from Gailey County (hence my name when the Norman aristocracy started it (last names) in around 1066) (Stafordshire?) I think from memory. Great great grandfather grew up there.

 

ah that explains it .
we have a saying here for people like you " he doesn't suffer fools gladly" ... its often used in the engineering trades as a compliment to describe someone who knows what they're doing and just wants the job done.
Any way It's all getting a bit off topic and strange .. my excuse is I'm sitting in the engineering dept in a large factory at 3:51am....and am very tired.
over and out.