New Brake Booster too sensitive
#1
New Brake Booster too sensitive
Hi,
My XJS has always had a fairly heavy brake pedal. There is power assistance when you run the motor but they required more foot pressure than my other Jags or any other Jag (old or new) that I have driven.
I got the local brake centre to rebuild it. I now have a light pedal, far too light! After starting the motor you can very easily depress the pedal to the floor. I took it for a short drive and the brakes now do virtually nothing for the first half of their travel and then bite extremely hard near the bottom of their travel. The car is not driveable like this, you could easily lock the brakes up without trying.
Any suggestions on what has been done wrong? Is there some sort of internal setting that can be changed? Thanks
My XJS has always had a fairly heavy brake pedal. There is power assistance when you run the motor but they required more foot pressure than my other Jags or any other Jag (old or new) that I have driven.
I got the local brake centre to rebuild it. I now have a light pedal, far too light! After starting the motor you can very easily depress the pedal to the floor. I took it for a short drive and the brakes now do virtually nothing for the first half of their travel and then bite extremely hard near the bottom of their travel. The car is not driveable like this, you could easily lock the brakes up without trying.
Any suggestions on what has been done wrong? Is there some sort of internal setting that can be changed? Thanks
#2
Join Date: Mar 2008
Location: Pacific Northwest USA
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I've never examined an XJS booster so I'm talking out of my hat here, but.....
Is there a pushrod between the booster in the master cylinder? If so, is it adjustable? Of, for that matter, is it even installed?
Also, vacuum booster typically have what's called a "reaction valve" which controls the amount of assist and thus sensitivity. Not sure if these are adjustable but, in any case, I don't think it would have anything to do with the excessive pedal travel. Just thought I'd mention it
Hopefully someone who knows something will chime in soon.
Cheers
DD
Is there a pushrod between the booster in the master cylinder? If so, is it adjustable? Of, for that matter, is it even installed?
Also, vacuum booster typically have what's called a "reaction valve" which controls the amount of assist and thus sensitivity. Not sure if these are adjustable but, in any case, I don't think it would have anything to do with the excessive pedal travel. Just thought I'd mention it
Hopefully someone who knows something will chime in soon.
Cheers
DD
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CaptainShakey (06-25-2021)
#3
I
Is there a pushrod between the booster in the master cylinder? If so, is it adjustable? Of, for that matter, is it even installed?
If it is like my 1985 non ABS, then it goes: pedal lever connects to forks on booster and booster bolts up to pedal casting. Then the master cylinder is bolted to the other (forward) end of the booster (or servo as we say over this side!)
There is a rod from the booster that needs to be engaged with the centre of the master cylinder.
DD
Is there a pushrod between the booster in the master cylinder? If so, is it adjustable? Of, for that matter, is it even installed?
If it is like my 1985 non ABS, then it goes: pedal lever connects to forks on booster and booster bolts up to pedal casting. Then the master cylinder is bolted to the other (forward) end of the booster (or servo as we say over this side!)
There is a rod from the booster that needs to be engaged with the centre of the master cylinder.
DD
Greg
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CaptainShakey (06-25-2021)
#4
I did the removal and installation myself, I just got the brake centre to do the rebuild. I haven't undone any brake lines so it shouldn't need bleeding. I also ensured the rod that engages the master cylinder was in place, and it was the same length as it originally was.
It seems to me the booster has been rebuilt incorrectly in some way so that it doesn't really push the rod until there has been a lot of pedal travel, and then does it very quickly and suddenly.
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CaptainShakey (06-25-2021)
#5
Problem solved.
The rod that protrudes from the booster and activates the master cylinder is supposed to have a little rubber disk (around 1/4 inch think) at its base in the booster. On my invoice its called a "Master Vac Disk". It's not required on all boosters but it is on the Jag. Mine was missing, whether they had forgot to install it or it had fallen out I don't know.
It all seems a little too simplistic but after installing one I now have normal brakes!
The rod that protrudes from the booster and activates the master cylinder is supposed to have a little rubber disk (around 1/4 inch think) at its base in the booster. On my invoice its called a "Master Vac Disk". It's not required on all boosters but it is on the Jag. Mine was missing, whether they had forgot to install it or it had fallen out I don't know.
It all seems a little too simplistic but after installing one I now have normal brakes!
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CaptainShakey (06-25-2021)
#6
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CaptainShakey (06-25-2021)
#7
Join Date: Mar 2008
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CaptainShakey (06-25-2021)
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#8
#9
The brakes feel good now. Just a normal amount of pedal force required. The car always seemed to have a heavy pedal but when it was the only car I drove day to day it just seemed normal. But after I got the X300 each time I hoped in the XJS the heavy brakes would annoy me.
I've also got a set of rebuilt calipers and hoses to install when I have time. The old ones are probably original and there is a small amount of pull under heavy braking
I've also got a set of rebuilt calipers and hoses to install when I have time. The old ones are probably original and there is a small amount of pull under heavy braking
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