Hi all! Many months ago (May, probably) I posted asking for confirmation of the proper rear diff for my '93 4.0, because it had been changed in the past and felt wrong, measured out to be a 2.88 unit. After confirming it should have a 3.54, I found one, but couldn't spend the time to install it because I was using the car while my normal daily was inop (Audi UrS4 parts aren't ever available locally).

The car has always had a left pull under hard braking after cruising for a while, which we assumed was a sticky piston. We were right... But the path to discovery could have been nicer. On a 7-hour highway cruise, we attempted to slow down for a tollbooth and discovered we couldn't. At all. Turns out the front passenger caliper had a stuck piston, had jammed the pad on, and overheated the brake "fluid" which was mostly air and rusty water. That's our fault, the PO had kept up on other fluids, so we assumed they'd kept up on brake fluid. Not so much... Bled it, some spacer trickery for the pad, and limped it home. This was the color of the fluid:


After that, I replaced all four calipers, pads at all corners, and did the rear diff while I had it all apart (which was covering the rear brakes in gear oil to boot). Found a leaky shock, but that's easy to deal with later. Then I discovered the real problem that caused our total brake failure- the rear brakes have no fluid pressure. While the rear suspension was out, I could get brake fluid out of the flex line by stepping on the pedal (none dripped out), but no matter what I did once the new brakes were on, nothing came through the bleeder, and the person in the car said the pedal never changed. Plus, the caliper pistons in the back never moved at all, they never contacted the pads.

So now I'm trying to figure out what exactly has gone wrong. Is it the ABS valve body? The master? There's another little valve for the rear line, right? Could that be causing an issue as well? I suppose it's also possible one of the new calipers is blocked internally, but I believe I cracked the flex connection and it didn't help either.

 

I believe the ignition key needs to be ON. I don't think the engine needs to be running, just key-on-engine-off.

It has been a LOOOONG time since I did the brake bleed on these cars but I think I remember it is similar to the XJ40 system.

bob

 

Now that's something that should be mentioned in the workshop manual... I didn't even think to try turning the key on. I guess the valve body needs to be powered?

 

Why do you have the inboard discs? I thought facelifts had the outboard discs!?

 

From what I've found out, the outboards only appeared at the very end of the XJS run, late 95's and 96s. (It's possible that when the rear diff was changed, the whole rear suspension unit was changed to one with inboard, but not only is the brake line layout proper, I bought the new rear calipers off the VIN, and got the correct hardware.)

 

I think it was 1994-ish when the change was made to OB brakes.

Cheers
DD

 

The procedure to bleed the rear brakes is to turn the key on and let the ABS pump send fluid to the rears. I had trouble with this as the ABS should only be run for about a minute or so. I used a cheap garden pump spray bottle to pressurise the system and then opened the bleed nipple.

Jaguar XJS Restoration : How to bleed Jaguar XJS teves ABS

 

Also, did you renew the flexible to the rear cage when you redid the brakes. These can collapse and block. if not, this is a must do, at the fronts as well.
Greg

 

I tried this trick, but didn't have an appropriate plug/fitting/cap for the reservoir. I guess this should be my next step, after the key on. Strange system that requires the key to be on for rear brake fluid, but then this is a car that needs the key turned on for the 4-way flashers to operate...

I did not, given it did puke fluid with the line open I figured it was clear, or that the puking cleared it out. If I can't get anything else to work, I guess I can try changing that line too.

 

HCl, I had read your post as if it would not bleed out with the unless you forced the pedal, sorry for the confusion.
Greg

 

Hi Hydrocloric

To Bleed the Rear Brakes, Turn on the Key but Don't Start the Engine, this will Start to pressurise the ABS Pump and should cut out when its done so.

But don't let it run for more than 20 Seconds to avoid burning out the Pump (As you already know)

If it doesn't cut out within 20 Seconds let it rest for about 5 Minutes and try it again for another 20 Seconds MAX.

All being well it should cut out either the First or Second Time you do this, in the unlikely event it doesn't, then you might have a Fluid leak somewhere or a problem with the Pump, though if that was working before then that is unlikely.

So at this Stage:

(1) The Pump Should have Cut Out in 20 Seconds.

(2) The ABS Warning Light Should also be Out

(3) The Ignition Should Still be Switched ON

(4) The Engine should Still NOT be running

(5) Make Sure the Brake Fluid Reservoir is kept Topped up at all times

(6) Have a 'Mate' Open the Left Rear Brake Bleed Nipple

(7) Push the Brake Pedal Down Gently and then Hold it down with the Ignition Still on for 15 Seconds MAX.

(8) Tighten Bleed Nipple While the Pedal is Still Down then Switch off the Ignition

(9) Wait a Couple of Minutes and then 'Rinse and Repeat' for the Rear Right Bleed Nipple

The Fronts are done in a Slightly different way.

Having Bled The Rear Brakes, give the Pump 5 Minutes to Cool Down and once again make sure the Brake Fluid Reservoir is kept Topped up, or you will have to start all over again.

This Time for the Front Brakes:

(1) Turn on the Ignition (But DON'T Start the Engine)

(2) The Pump and ABS Light should go out in 20 Seconds (No More to avoid Pump Overheating)

(3) Once the Pump has Cut out (20 Seconds MAX) Leave the Ignition ON (DON'T Start the Engine)

(4) Bleed the Front Brakes Conventionally (With the Ignition ON) Using Full Smooth Strokes of the Pedal until all the Air is out and then do up the Bleed Nipple.

(5) Switch OFF the Ignition, give the Pump 5 Minutes to Cool down, then 'Rinse and Repeat' for the other Side.

(6) Check Brake Fluid Reservoir Level

(7) Turn ON the Ignition and the Pump Should cut out in less than 20 Seconds and the ABS Light should also go OUT.

(8) The Brakes Pedal Should be feeling good and if so Start the Car and then go for a Slow Test Drive round your Yard.

Credit@PTJS

 

Well that's kinda true, while disconnected the rear line didn't have any flow until I stepped on the pedal, at which point a copious amount of fluid came out, but it still didn't drip like the fronts did.

Thanks! That's a hell of a procedure for just bleeding brakes. I'll give it a go the next time I'm with the car. Interestingly though, the rears on my car have only a single bleeder- the line runs to the passenger caliper, then the driver, then the bleeder.

Also the front bled out conventionally with the key off, interesting that the key should be on for them too.

 

There should be a bleed screw on BOTH rear calipers, otherwise the one without will not bleed properly. It may well be broken off, I am sorry to tell you. Also, I am pretty sure that the hazard light should work without the key being turned on, unless the facelifts are different from all other cars, so if yours do not there may be a fault. Still sounds like the rear flexible could be faulty to me!
Greg

 

Hi Hydrochloric

If both your rear Calipers are connected together, then the job is even easier because you only have to do it Once instead of Twice (still only 15 Seconds though!)

So as Greg says, you will have to Undo the Bleed Nipples on Both Calipers This is before you Start Bleeding them, as the 'Bleeding Part' happens on The Single Bleed Nipple where Both the Bleed Nipples are joined together as One. (If you see what I mean)

When that has been done and the Ignition has been Turned OFF (After 15 Seconds MAX)
Then do up Both Bleed Nipples on the Actual Calipers themselves, as well as the one on the end of the 'Single Line' that joins them Together and everything should be 'A' OK after that.

The reason that there are what appears to be Two different systems working in Tandem Together is a Built in Safety device.

So if your ABS pump decides to pack up, then you will/should Still have Braking on the Front.
If that makes any sense, because your Front Brakes are the Conventional ones.

 

There was a change made to the rear brake pipe system where the T piece was deleted and the rear calipers were in series. I have also seen remote bleeders on the IB rear end So your singe bleed nipple must be after the last caliper.

The ABS is a strange setup. The front brakes work normally without any assistance until the ABS activates. the rears however are run from the pressurised ABS pump. The reason for this as OB said is a fail safe measure for the front brakes.

 

The bleeder better not be broken off, the calipers are brand new! I actually took one out to put the line that crosses from one caliper to the other when doing a complete diff swap and new calipers, nothing broken.

Okay, finally this is all making sense! That is how my rear lines are set up, flex line onto the rear suspension unit, to the pass caliper, to the driver's caliper, which has the actual bleed nipple.

That explains why it puked fluid the first time I hit the pedal (while the line was disconnected) since it had some pressure left over, but not the next time since I hadn't cycled the key.

Excellent, now I know exactly what needs doing. After this I just need to figure out why the A/C died...

 

Just felt I should update:

Rear brakes bled no problem, knowing the key needed to be on and the pump charged. I thought it wasn't going to work, then it spit a little bit of fluid before firing a deluge out.

Anyway, with new brakes, new (correct) rear diff, the car is much happier. It no longer feels like I'm begging it to get moving, and now I can believe in the brakes again.

Mostly. On sudden brake application there's a momentary rightward pull. Not enough to steer the car, but very noticeable. Oddly, before changing all the brake, the car would pull leftward on sudden brakes. I'm not sure what to look at for that, but for now winter has come and the car is in slumber.

 

Congrats on getting them done. This slight pull you mention is a classic sign of either a slightly sticking front caliper, or unmatched/sticking front pads, or both. The front pads can quite easily jam in the caliper. So I would look at that first.
Greg

 

Hi Hydro

If your Car should pull to one side after a sudden application of the Brakes, then that could be a Classic Sign of a Blocked Valve or a Broken Wire in the ABS Valve Block.

A Blocked Valve in the ABS is usually the result of either not renewing Brake Fluid on a regular Basis.

Or Levering the Brake Pads back into the Calipers, when for instance replacing the Brake Pads, without having the Bleed Nipple open (This is the Kiss of Death on an XJS)

As this sends Brake Fluid the wrong way up the System, where the Microscopic hole in a Valve (or Valves) can get Blocked up with a Microscopic piece of 'Crud'

Hence the Sharp Pull from one side to the other.

Or it could be a Broken Wire in the ABS Valve Block which can produce exactly the same result.

Or a malfunctioning Wheel Speed Sensor.

They are not really Wires but more like Ribbon Connectors, that get very Brittle over time, so that a Jolt when driving the Car can break one, as they are very Fragile.

Spares are no longer available, so you would have to rebuild the Valve Block yourself, or replace the Complete Abs Master Cylinder Actuator with one from a Parts Car.

If you decide to rebuild it, there is no guarantee it will work although it could be worth a try.

Its really not that hard to do but more just very frustrating when all you want to do is use the Car.

If it comes to replacing those Ribbon Connectors, its best to replace them with Wires but unless you can Solder like a Pro, its best to get a

Radio Shop to do the Soldering for you, as if one of those Wires comes off then the Car could just fly off the Road!

This is the Nuclear Option, as hopefully this will turn out to be a Stuck Brake Pad or Caliper like 'Greg' Said.

But if not I can show you how you can hopefully fix it.

 

Hi Hydro

In Answer to your question:

Also the front bled out conventionally with the key off, interesting that the key should be on for them too.[/QUOTE]

Should the Key be ON or OFF when Bleeding the Front Brakes?

The Answer to that is: 'It all depends'

Theoretically you can Bleed the Front Brakes with the Key OFF but a lot depends on how much Brake Fluid you put through and how much pumping on the Pedal it took.

The idea of Keeping the Key on 15 Seconds Max when Bleeding the Front Brakes, is to Regenerate the Brake Pump to keep it up to pressure, so it cuts out before you start doing the other side.