1990 XJ....I have replaced the accumulator twice AND the pressure switch twice to try and fix this problem. I thought maybe the first accumulator was bad one out of the box so tried a second new one along with a second pressure switch and the exact same brake problem continues. I'm about to pull my hair out.

Start the car, the pump as it should then builds up pressure until it shuts off when it reaches max pressure, both the ABS and the "brake" warning lights are off. Then press the brake pedal ONE time and hold it down, the brake pedal feels normal height and resistance engaging the brakes... then the brake pedal starts slowly sinking to the floor, at the same time and immediately while the pedal is sinking you can hear a hissing noise exactly similar to a vacuum leak noise (but can't be because no vacuum involved). The hissing continues until pedal bottoms and wont sink anymore or you let up on the pedal. The hissing noise must be a rapid loss of fluid pressure because the pump comes back on soon after pedal is almost to the floor. The next pump of the brake pedal and all other pumps of the brake pedal after are the same result EXCEPT when the brake pedal comes back up it hesitates for a second and then releases with a smack up against the bottom of your foot. When driving it eventually the brakes go out almost completely if driving with rapid use of brake pedal.
I repeat... I have replaced the accumulator twice and the pressure switch twice along with the ABS computer and had it properly flushed and bled by a shop and I still get the exact same results. I am considering replacing the whole damn unit that the accumulator and pump attaches to, what else can I do? Does anyone have any other suggestions???????????? Thanks, RJ.

 

Did the problem start before or after the shop bled and flushed the brake system? And is the shop familiar with Jaguars and the correct procedure for bleeding the brakes?

 

Yes it started before the brakes were bled, I had it done at a shop that I know well and know Jags, I've had the car since 94, they too are not positive what it is and I'm not looking to make a large shop repair bill over this, its not a primary vehicle. Over the years I have kept it running and fixed just about everything that does not require a lift. I have gone through several accumulators in that time and familiar with what a bad one does and replaced them, but this time its clearly something more. From here all I have is troubleshoot by parts replacement, I was hoping someone on here has experienced what I'm describing and could tell me what their fix was. Although it appears to be working correctly I will see what a new pump unit does for it, if that does not fix it then I will have to try another master cyl ABS modulator assembly. Love the car, hate the brakes. Why didn't they just stay with vacuum assist...pfft.

 

You must think of hydraulic braking system and hydraulic boost system as separate because they are. The symptoms of pedal dropping to the floor are classic master cylinder internal leak symptoms. This is true weather you have a vacuum or hydraulic booster. Consider changing the master cylinder. On a car this age its due in any case. The accumulator has nothing to due with pedal travel. Its there to simply ensure you have enough residual booster pressure during a rapid pedal hit as would be needed in a panic stop or during a engine failure. I'd also put a mechanic stethoscope on the master cyl and ABS pump and listen for the hiss sound. That's where issue will be.

 

Yes I do see them as separate, only common factor that I can determine is they both get fluid from the same reservoir, and this is one of the models the leveling system is totally separate. Thinking about it I believe you may be correct that regardless of what is happening with the boost system the pedal should not be dropping to the floor. I will do some listening for the hiss like you recommended and try and isolate it, it may be the boost system trying to provide for the sinking pedal and why the pump is coming on prematurely. Thanks!

 

I agree with icsamerica that the pump may not be the problem. Pressure is being built up and the system is ok until you apply the brake pedal. That makes me think it is in the unit directly attached to the brake pedal. There may be a valve sticking within the unit. Not sure if this unit(control valve may be what it is called) is repairable or not. Cary

 

It could also be reasonable to relate that its a sticky control valve that is also causing the problem that exists when releasing the brake pedal. On its return to the rest position a lot of times stops in its travel for just a second and then suddenly releases.
I presume its more about the control valve hanging up then the part of the booster assembly that is sandwiched between the control valve and the brake pedal housing. The Booster section is clearly more simple, the control valve has more things to fail. It looks like logic says that may be the next item to try.
From what I am finding on web regarding ones for sale they are being identified as ABS Brake Master Cyl Modulation Assembly, and most seem to include the booster section in front. New cost is madness, used one is around 150. I'm not seeing a kit to rebuild them. Thanks to all for the help.

 

bringing back alivr and old post.
where you ever able to find the problem?
I seem to be having the same issue.
a hissing when I press the pedal, followed by the pump kicking on then, after a bit "brake" light then abs lights. and then to the floor ffollowed by a pumping noise I can feel in the pedal. then the pedal goes hard and kicks up and I have to hold it to the floor to keep from creeping forward. replaced ball. now the car stopps great. I just can't hold it.
Im at a loss on what to do.

 

I also agree with icsamerica. What describe is a classic symptom of a MC leaking internally.

 

See the XJS forum,about similar brake issues,just a few threads back,includes a link,with all the Teves MK2 systems.Nice read!

 

Golly Gee there Jim, gotcha self an XJS after all, late present eh?

I see it's a real antique one though, early car alright, an 1886 model

 

Eeeek,Yup new toy,Birthday present!

 

Well happy birthday there man and many more of em (birthdays that is, not XJS's!!!)

 

Well an update!!!

I found out the actuator the thing before the front "master cylinder" was having a leak inside of it some how. that was causing the pressure fluid from the accumulator ball motor etc... to somehow bypass as i pressed the pedal. you could hear the pressure just hissing as i pressed the pedal.
I test a fellow members car, and you hear nothing.
so i found a whole pedal master cylinder assembly from eBay, stuck it on bled the system and BOOM i now have normal if not very sensitive brakes now
ill try and get some pics up this week.

 

Thanks for the update, Shawn! We'll look forward to the photos!

Cheers,

Don

 

Good job Shawn.

 

here is a pic is the bad part.
it was leaking Inside there.