Hi,

I just picked up a 1990 Jaguar XJS V12, and the main issue is the brakes are completely non-functional. The pedal drops to the floor with virtually no resistance, and there is NO braking whatsoever. Tried some diagnosis today:

With key off, pumped the brake about 20 times - the pedal got stiffer, but would never hold pressure (would still drop to the floor when pressing on it with a little pressure)

When I key on after this, the hydraulic pump runs for about 35 seconds. During this time, the ABS light is on... the ABS light goes out when the pump stops running. From todays research, I think this tells me the accumulator and pump are OK??

As SOON as I touch the brakes, the pedal is soft at this point, the ABS light will come on again, and the pump will start running. Brake pedal goes to the floor.

So I'm getting mixed thoughts on what this could be - master cylinder, accumulator, both? I wouldn't think a bad accumulator would cause the pedal to always sink to the floor??

Fluid is full by the way!

 

If the brake level is not dropping then my first guess would be the master cylinder seals. Wouldn't expect the accumulator to make the brake pedal go to the floor. Try it with the engine switched off, if the pedal goes to the floor and there's no loss of fluid then the master cylinder seals are the most likely.

 

Yep, even with the engine switched off, it is impossible to get the pedal completely firm - it will still drop to the floor. No external leaks, no loss of fluid.

Ok so next question, where to find the seals / replacement master cylinder? The TRW ones state 'none ABS', others don't specify one way or the other. What is the correct part for an ABS equipped car?

 

I can't help much....never had to reapir this type of ABS, personally.

Accumlator failures are common. At $80 or so it's fairly painless as guesses go.

You don't have a master cylinder...not in the conventional sensor. It's all part of the "pump-actuator" assembly....about $3000.

No doubt there are outfits out there to rebuild these units.

Some have retro-fitted conventional brake hydraulics.

I'm sure there's good info in Kirby Palms book

Cheers
DD

 

Wow, just found Kirby Palms book, amazing information in there!

First steps will be a full flush and bleed over the weekend, see where that takes us.

 

Ok, an update for anyone that's following!

Did a full flush and bleed with Dot 4, drained some AWFUL looking brake fluid out of the lines and calipers, as well as a little air, so we were hoping it would be THAT simple....

But no - although the pedal had a little more resistance, it would still drop to the floor, and it also felt like it would return pretty slowly.

So, feeling pretty downhearted, we did a little headscratching while looking at the schematics from the Mitchell's service and repair manual (which is great by the way - photocopied the relevant parts from the copy our local library has!), at the same time dreading the thought of having to remove and disassemble the master cylinder assembly.

Although looking pretty complex, the Teves ABS system is actually fairly simple once you get your head around it. I had a thought - the ONLY way I could see the pedal would be able to drop to the floor, is if the 'normally closed' ABS solenoids were leaking fluid past. So, as a long shot, I put 12v directly onto each solenoid to open and close them a number of times, hoping one of them might just be stuck.....

BINGO!! Hard brake pedal!!! Did some cautious checks, and took it onto gravel to exercise the ABS system again, get the new fluid through there etc, and it's working PERFECTLY! Also ran through the checks for the accumulator, and that is working perfectly also, so no need to change that.

So, two lessons I think:
1: Change your fluid regularly! I suspect the old, bad fluid and a little used car were the main reasons that solenoid stuck in the first place.
2: Use your ABS. I can't help feeling activating the solenoids once in a while will keep them from sticking, and will cycle fresh fluid through them. Easily done on gravel or snow.

 

Great result there. One to remember in the future.

 

Very, very, very well done !!