S-Type squeaking brake pedal

[gregp]

For some time now, my S Type Sport Diesel has suffered a squeaking brake pedal when cold. This is a very loud squeak/squelch noise, coupled to extended pedal travel, which disappears once the car is hot, probably due to heat soak from the engine bay. My dealer is unable to rectify, or even replicate the fault, even when the car is left overnight, but it is there every morning when I get in the car! The servo pipe has been replaced but made no difference. Has anyone had a similar problem, and if so, what is the solution? It is driving me mad, so any help would be much appreciated.

[Jaguar steve]

A squelch and mushy pedal - especially when starting from cold suggests either the servo is slowly loosing vacuum, the master cylinder is leaking internally or there's a very small amount of air trapped in the brake system somewhere. Or if it's more of a squeak then a soggy squelch a shot of WD 40 on the pedal pivot may be all that's needed.

Try to replicate the noise by swiching the engine off and steadily pumping the brake pedal up and down several times to dissipate the vacuum in the chamber. You should feel the pedal become progressivley firmer as you pump. Then start up with your foot lightly resting on the brake pedal - with steady pressure you should feel the pedal drop slightly as the vacuum builds then become firm.

When was the brake fluid last changed? Can you push the pedal all the way to the floor with a long steady push? Does it only happen the first time you brake or does it take some time before it dissapears. Was the valve in the servo replaced as well as the pipe?

Have a new valve fitted to the servo and change the fluid if it needs it may be the best place to start. If you can get the pedal all the way down with steady pressure you'll need a new master cylinder too.

[gregp]

Thanks Steve. A new master cylinder was fitted some months ago and the fluid changed. I will try the tests you suggest and report back.

[Jaguar steve]

gregp said:

Thanks Steve. A new master cylinder was fitted some months ago and the fluid changed. I will try the tests you suggest and report back.

Why was it changed? Was fluid leaking into the servo chamber? Is this when the problem started?

[gregp]

Not sure, I need to check with the dealer. I can see your point though, thanks.

[Jaguar steve]

If it's had a new master cylinder there's a chance they may have not bled the brakes properly afterwards and there's a tiny amount of air in the system somewhere. If it's been like this since the work and was ok before then take it back, tell them you have not been happy with the brakes since the job and ask them to bleed the brake system throughly again.

Gas will compress under pressure, Fluids will not. Brake systems in normal road use will tollerate a few tiny bubbles of air - the usual symptom is a mushy pedal, somtimes with the squelching noise you've described, and a bit too much travel - but nothing too noticable, until that is, you heat the brakes up after a long descent or several hard stops. Then the air trapped in the system expands with the rise in brake fluid temperature, the pedal travel increases before any braking effect and you could end up with a serious or total loss of braking.

[gregp]

Thanks for the comments.

Success at last! Finally got the dealer to drive the car from cold to demonstrate the problem, and I now have a new servo! After initially saying it was only a squeaking brake pedal linkage, they finally decided that the fault was within the servo. Fortunately, I had extended the warranty, so the repair was covered! A result!!! Interestingly, the brakes now feel like those on the many courtesy cars I have had over the last 33 months!!