Hi guys,

I am new to this forum, although had my Jag fo a while now. It is an XJ6 (XJ40), 1993, 3.2l, UK model.

I have just replaced the headgasket on it as the engine overheated after alternator got stuck. It runs fine now but I have a problem with the brakes.

When I am driving it is kind of OK, but after 10 seconds of holidn pedal down the red brake light comes up, and after a while ABS light. If I release the brake pedal it disappears after apro 15sec.

When I waiting for the lights to change on the crossroad, and have the brake pedal down, after arround 30 seconds the brakes no longer hold and the car starts moving. I can hear buzzing and pedal slightly vibrates.

I was given an advice that it could be brake accumulator, or air in the system. I was thinking about brake pump but I am not really sure. Checked the book from FAQ but never get a chance to test it on car yet.

Do you have any ideas what could that be?

Thanks for your help.

Tom

 

also when pressing pedal down I can hear 'hissing' as if the air was going out from somewhere... maybe its just air in the system?

oh and the brake fluid level is higher than the max point

cheers
Tom

 

I would suspect the brake accumulator.

 

Thanks, any idea where to get it from? Cheaply

 

The answer is above the question.

 

Don't know about where in the UK. We sell them here:

http://www.motorcarsltd.com/JLM1907.html

 

UPDATE:

I have changed the front brake pads as one was knackered, renewed the brake fluid and front brake lines.

Symptoms:
Car no longer releases the brake, it brakes fine, but after a while the pedal becomes rock hard, lights come up (brake warning and ABS) and the pedal is making a buzzing noise and is shaking a bit.

I have replaced the brake accumulator with spare one and it never fixed the probelm.
Any other ideas what could cause that strange behaviour?

Regards,
Tom

 

Are you sure your spare is good? I got my XJ6 with bad accumulator(and the pump fried because of that) bought a new unit with a bad accumulator so brakes fine but get exact same symptoms as you have.

 

To be honest the brake acumulator I have fitted is as old as the other one (if not older). Could that damage brake pump? I am just trying to find the eact fault as the price for new brake acumulator is horendous!

Thanks for your help
Tom

 

Yes if the ball dosent have a good diaphram there will be nothing to keep applying pressure to the fluid inside it. The pressure switch I assume just tells the pump when it needs to pump more pressure to the accumulator.

My car needs a new accumulator(I am going to buy it in a week or so) and the symptoms I have are I hear a click intermitently and when I apply the brake. This is a noise that happens when the brake pump turns on. It never runs constantly just fills the ball up and refils it when some pressure leaks out I assume but since the ball looses all of its pressure so fast because the diaphram is bad the brake pump needs to pump every time I apply the brakes.

If you do drive like this be very conservative with your braking. Don't pump the brakes and don't apply them very fast if you can avoid it. When you aproch a stop sign for example apply brake early and hold it in one possition, this will hold the pressure you already have without making the pump turn on more than once or twice per stop. So the pump does get a lot more use but it wont run and overheat itself and die. I think the number I read was for every 2 minutes the pump runs it must rest for 10 or it will overheat.

This said get a new accumulator because when I bought my XJ the accumulator and pump was bad(the pump was burnt out) I beleve because the pump just got over ran because the accumulator finally failed completely and the previous driver was probably quite aggressive. I found new accumulator on ebay for 199. So as soon as you can get the cash buy it because if that pump fails your brakes are all manual(and they don't work very well that way) and now you need to get a whole new ABS brake assembly which will run beween 300-600 bucks unless you can find someone selling just the pump.

 

Thank you very much for the explanation! I will try to get the brake acumulator as soon as, just worried that it is going to turn out that the fault was not because of it. But from what you are saying I guess it must be.

Just one more thing, how can I check if the brake pump have not died on me yet? The car still brakes but just to be 100% sure

Cheers,
Tom

 

In my experience, for what it is worth. A failed brake pump or servo on any vehicle will give you a rock hard pedal and you will be gritting your teeth from the force you need to use to stop the car.

A general quick check of your pump/servo can be achieved by the following.

1. Engine off, press the pedal 4 to 5 times to lose the pressure.
2. Hold pedal pressed down and start the engine, as the engine starts you should feel the pedal 'give' as the vacuum builds.
2a. Release pedal.
3. Run car for a few minutes 3-4 then switch off.
4. If you now press the pedal you should detect a hiss. After 4-5 pedal presses you will get no more hiss and the pedal will be hard.

You just proved that your pump is working (at least for now).

 

Or if you have a friend, have him press the brakes with the car on while you are looking at the pump.

 

I have replaced the brake master cylinder and it solved the problem. Now the warning lights do not come up and it brakes fine!

Thanks for all your help!

Cheers,
Tom

 

I am curious what was brg with the master cylinder to cause the brakes to act like that.

 

I am not going to rebuild it now but I guess it was something to do with the pressure it was generating - not enough to hold the brakes... maybe sensors? I dont know. Glad it is sorted tho!

Cheers
Tom

 

Well glad you got it sorted.

I've seen this on some more basic Brit cars. Duff seals in the master cylinder. You get brakes when you first hit the pedal but then the fluid gradually drains past the pistons causing the brakes to fade away.

But usually the pedal falls away as well.

Odd that your pedal went hard.