Hi I have a 1997 XK8 that after a winter lay up, when i drove it out of garage it went 30 yards and locked up.(I think i might have touch brakes at this point) rear wheels drive but car digs itself into gravel drive. Jacked up all four wheels which rotate in neutral, ignition off. But have now found that as soon as i turn ignition switch on, foot brake goes down and all wheels lock. So brakes work. I have removed vacuum pipe from brake system and brake now comes off. What is likely to be wrong. Failed or stuck cylinder?????

 

Try removing fuses 4, 16 and 18 from the engine compartment fusebox. Put the vacuum back and try again. This will disable the electronic anti-lock braking, which could be malfunctioning.

Please let us know the outcome.

 

Thanks Whitexkr.
tried that with fuses and also disabled the ABS at source, but with vacuum back on it still locked the foot brake.
wounder if as i had not used the foot pedal for some time the plunger was stuck part down and sending a signal to vacuum assist and it hence comes on. tried pumping the brake pedal and pulling it back but no good. I note the orange warning light is on dash panel but ABS light goes through its warm up and stays off.

 

Are you sure the emergency brake is off? Try pulling it up pushing the button and letting it down. Just a thought..

 

Rick L, thanks tried that, a few times, and it certainly releases the brakes when ignition is off, but as soon as switch the ignition all wheels brake lock. As said as soon as ignition is turned on brake peda , on its own goes down.

 

Sounds like the brake booster has its vacuum inlet valve stuck open. Frankly I've never heard of anything like this before but in my view that is only answer. I suggest you remove the booster and check it out.
Good luck,

 

+1 I've also never heard of anything like this before . I am assuming that you are suggesting detaching / plugging the vacuum line to the booster to test first before removing the booster unit itself.

 

Offhand I'm not sure what the vacuum line is like, could it potentially have delaminated inside so that it effectively becomes a one-way-valve (this can happen with flexible brake hoses and cause the fluid to trap and the piston to lock on)?
It's a long shot, but the only thing that comes to mind.

 

Thanks all, I need some time to investigate, unfortunately off to Italy for a few weeks, this week end, but not now in the Jag! what a shame road trip across the Alps and in those tunnels with that beautiful roar (soft Top)
However, The problem appears still, when the vacuum hose is taken of the brake servo there is great vacuum pressure, if i put it back near the hole, but not in it, the bakes go on again. Should there be constant vacuum pressure in the hose?
I notice also that the LCD display on dash has now totally gone, it had been getting darker for the last few years. Is it possible to get it rejuvenated? Or not worth it, as not necessary for legal road test?

 

Hi All, sorry but so I am sure I understand your helpful comments, being from England, is Brake Booster- the Brake servo and is Electronic anti lock brake -the ABS. i assume emergency brake is hand brake!

 

Agree with what's been said - if the pedal "self-pushes" - it can only be doing so by vacuum and if the engine isn't running - it must be residual vacuum.
But ............ I assume you have pressed the pedal enough times already to have released any residual vacuum already in the system.
Have you tried REALLY pumping the pedal with ignition (and engine) off ??
Is the pedal hard or spongy - is there any change as you pump ?
Just thinking aloud and clutching at straws !!!


Edit - I don't think a working odometer is required for the UK MOT. They will probably just put "unrecorded" against the mileage box on the ticket. It's not a "safety" issue, and therefore not tested.

 

Devon Dave
the peddle self pushes as soon as engine starts up, and also soon after it has been running on ignition key turn on, so that must be residue vacuum, so it must show the vacuum is working well, just that the system thinks it needs the brake to work with some sensor saying so. car seems to roll when in neutral so i cant believe it is the cylinder part stuck on. Not sure what White Bear means by the vacuum inlet valve stuck open. Where is the valve, as said vacuum seems continuous out of vacuum tube.
Yes have tried pumping brakes , ignition on and off, feels perfectly normal when ignition off slight movement with slight hardening of pedal, but seems very normal.
Ps I am Somerset David, so nearly local.

 

To clarify how the brake servo (booster) works:
Your booster (brake servo) is always under vacuum when the brake pedal is at rest. There is diaphragm inside the servo (booster) with a valve that allows the vacuum to go both sides of the diaphragm. There is another valve connected to your brake pedal push rod that will, when the brakes are applied open to allow atmospheric air pressure to enter the servo and at the same time close the valve in the diaphragm so atmospheric pressure is only felt on one side of the diaphragm. It is atmospheric pressure that pushes the diaphragm toward the vacuum side, and at the same time pushes on the brake master cylinder, giving you the brake pressure boost you need to apply the disc brakes. (Servo's or boosters are only needed on cars with disc brakes)

My point is you have the symptoms of a classic situation where your brake pedal is being held down, letting atmospheric air into the servo, and therefore applying the brakes. I know you've tried to pull the brake pedal back up, but this time get under the dash and look. If your brake light switch or cruise control cancel switch is mounted under the dash, and is now stuck in an extended (open) position, or bent so that the brake pedal cannot return to its full off position. To prove this check to see if you have any brake pedal free play, it should have at least 1/4". I'd place a small bet there isn't any.
If this is the problem it will be an easy fix, so that drive through the Alps may be on after all.
I envy you that.
Good Luck.