The car came with several brake problems, first the ABS light was on due to a bad front wheel sensor, this took care of the problem and the light has been off since.
The problem that I cannot seem to get rid of is that when the car is started the pump comes on like it should and initially runs for a min or so and then shuts off like it should. Then with even one single pump of the brakes the pump comes on for a short while until it builds up the required pressure and then shuts off. The car can sit and idle for however long and the pump does not activate unless I pump the brakes just once and immediatly the pump activates, its my understanding that I should get a lot more pumps from the pedal before the pressure is lowered enough for the pressure switch to activate the pump. Where is all my pressure suddenly going in one pump? To try and eliminate the problem a bran new accumulator, a pressure switch, an ECU, and a complete flush and bleed has been tried, yet I continue to get the same exact results. Also when driving, especially in heavy traffic where the continuous need of pumping of the brakes eventually leaves you with almost no braking. Can anyone suggest what this may be, I don't know what else to throw at it. thanks..

 

Sounds like you have a leak somewhere in the system. Just to clarify, the mineral oil system (accumulator, pressure switches and the like) is just there to 'boost' the normal braking system in the same way that a vacuum booster does. If this fails then you still should have braking capabilities albeit with reduced performance.

Your mentioning that you have virtually no brakes in heavy traffic suggests that I would be investigating brake lines and master cylinder.

Also, have a look under the dash and see if there is any fluid leaking from your brake pedal box?

 

Maybe you need to change the pump. Have you used the correct mineral oil. It is green and is from a castrol bottle. JLM 9886 is the part number. It is made for that system. Are you using new parts? Are the pressure switches wired properly? That hydraulic system is a bear to figure out. I spent three days diagnosing a car and finally determined one of the pressure switches was bad. I was still wet behind the ears then but I took the time to figure it out. I think the hydraulic system book is here on the forum somewhere. That would be a very helpful book to find.

 

You must be registered on Jag-lovers.org to read this but here is a link for a lot of good test for that system. Go to the link and scroll down to suspension system.

XJ40_E_BOOK

 

The S 73 Hydraulic guide + others is available here.

Follow this link and scroll down to XJ40, click through and you will be taken to our XJ40 downloads. There is even a full Haynes available.

HTH

 

Thanks for all the good info. I have no leaks in the system, all reservoirs are staying full. As I suspected after reading the hydraulics publication xj40 my brake boost is the one that is 1990 and on, or one that the diagram that is showing it does not have any feed to the brake boost from from the mineral oil system, and clearly I cannot find any hydraulic lines feeding to it. It appears the 90 onward system is not only using brake fluid for braking, but its also using the brake fluid from the pump for boost assist. This is where everything seems to stop in the way of information, apart from replacing the accumulator, pressure switch and ECU what else in this design could be causing the pump to come on with only one pump of the brakes? It does not have a conventional master cyl like the pre 90 system does, everything is different above the ABS valve block and below the brake fluid reservoir. Throwing parts at this is about all I can do to continue my effort since there seems to be no source of troubleshooting this system. I am about to try finding a whole new valve block and pump unit, or in other words replacing everything except the brake fluid reservoir and see if that cures my problem, because replacing the obvious is not fixing it.

 

Well Mark is waay more experienced than I am. I would have said 'sphere' but you say that you have installed a new one and correctly bled things.

I have a duff sphere, and have exactly the symptoms you describe.

You don't mention what type of rear suspension setup you have, SLS or modified.

 

That's right. Second design of that system. I forgot they took the brakes out of the SLS stuff. Hmmm.

 

You have a faulty accumulator. You should get 20 to 40 brake applications from a fully charged sphere with the KEY OFF before the pedal goes HARD.

You should get 8 or 10 brake applications from a NEW accumulator with the KEY ON (engine running) before the pump engages to recharge.

A NEW OLD STOCK accumulator might have lost the nitrogen charge in the years of storage. (just 'cause it's new doesn't mean it's good)

bob gauff

 

I am mostly in agreement, considering the characteristics of the problem are exactly what a bad accumulator would produce, you may be right and the new one I have is crap. Trying another new one is worth it at this point. Just wish they weren't so darn expensive.