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

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

Assuming the brakes are bled properly then there is something amiss in the pedal/booster/master cylinder linkage. Take it back and tell them to fix it. A non-ABS car should have max 1/2 inch of pedal travel, engine off, before the brakes go hard.

Greg

Sorry, I should have clarified in my original post. I do have a hard pedal with limited travel when the engine is off. But when the engine is started the pedal goes very soft and can be pushed to the floor easily.

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.

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!

After some googling it appears the part is known as a 'reaction disk"

Good work !


Cheers
DD

How do you like the feel of your brakes now?

I definitely have to use just a bit more force than I like at stoplights, but the engine rpm appears normal. I've wondered for a while if there was a brake adjustment for this.

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