Front Brake Calipers Engaging Separately

XJ-S
1

I have a 1994 XJS straight six in the shop now with peculiar brake issues. It all started with lights on the dash, typical old accumulator issue. Replaced accumulator and all was well… sort of.

No ABS or Brake light on the dash. Old fluid was completely flushed out and bled. Rear brakes were bled per service manual by turning on the ignition and using the brake pump to pressurize the fluid.

On a test drive, the brake pedal is firm, stopping power is great. However, approximately every 10 applications of the brake pedal, the pedal sinks to the floor, and the car is jerked from side to side as you first feel the passenger front caliper engage, and subsequently the driver front a moment later.

I spoke with a gentleman in Connecticut that owns a Jag Specialty Shop and based upon this, he recommended replacement of the master cylinder to repair the problem. He said he had seen this exact issue ONCE before.

So onto the master cylinder replacement. First replacement was a dud. So had to install a second “remanufactured” master cylinder.

Bled out perfectly, again with the procedure stated in the service manual, also bleeding the brake pump itself.

Everything seemed wonderful, no lights, firm pedal, brake pump would only engage when needed.

Go for a spin, exact same issue. Sinking to the floor about every 10 applications of the pedal.

Coventry West wants us to install another master cylinder… I do not believe the master to be the issue. We have also replaced the proportioning valve as PM but nothing changed.

The only other thing I can think of is the brake pump itself… If it is weak, every 10 or so applications, there is not enough pressure in the system. Therefore the sinking to the floor and delayed application of the front calipers.

Another note, after normal driving the front brakes will be around 250-280 degrees F and the rears at about 150.

As it seems, the rears are not doing as much work. When the car is on a lift and the pedal applied, all 4 wheels are locked and cannot be moved.

Any insight is much appreciated.

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The most likely culprit was the master cylinder, but you’ve addressed that. The second most likely culprit is the ABS system somehow deciding that the brakes are locked up and releasing them for you.

My recommendation, as always, is to pitch that Teves III system in the dumpster where it belongs and either upgrade to the Teves IV or retrofit the pre-ABS system. The Teves III is dangerous, as you are learning.

If you’re unwilling to actually fix the problem, one other thing you might try is to get the car onto a gravel road and lock up the brakes a few times, thereby exercising the ABS valving. Who knows, maybe there’s a chunk of debris in the valves somewhere. Locking the brakes actually causes a flow of brake fluid, from the reservoir to the pump through the master cylinder and ABS valves and back to the reservoir, so hopefully it’ll clear any crud out. Brake operation without ABS activity does not flow any fluid, it stays in place and gets stagnant and gradually accumulates contaminants.

You might also check the wheel sensors, especially on the front wheels. There are reports of junk getting on them and causing all sorts of odd behavior. They usually don’t need replacement, just cleaning.

A brake pump completely inop should not cause the pedal to go to the floor. It would only result in you having to push about 4X as hard to operate the front brakes. And the rears wouldn’t operate at all. But the pedal would be hard.

A brake pump that’s foaming the brake fluid could cause an intermittently dropping pedal, though. Are you using the correct brake fluid?

1 Like
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Thanks for the reply.

I know the brake pump is operating, you can hear/feel it turn on when the ignition is on and you crack a bleeder. The pump works and shoots out fluid at pressure.

My thought was that a weak pump would cause this issue. As in, it cannot keep up with the demands of the hydraulic system, hence the loss of pressure and sinking pedal. It is a thought.

I’ve only ever used DOT 4, says right on the reservoir.

I tried your recommendation of cleaning sensors, locking up the brakes, etc. Even tightened a loose front wheel bearing that may have attributed to wonky braking.

No results thus far. Same thing is going on.

This is a customer’s car so some decisions will have to be made before going the route of gutting the system for pre-ABS.

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I doubt this is the problem but have you tested how many pumps of the pedal you get with the system fully charged but with the ignition off?

5

No, it’s not. As I said, even if the pump failed completely the pedal shouldn’t go to the floor.

I suspect the ABS valves are intermittently opening to release pressure. If so, there are at least four possible reasons for that to be happening:

  1. Junk in the valves.

  2. One or more of the valves is malfunctioning.

  3. A sensor is dirty or malfunctioning.

  4. The ABS ECU is malfunctioning.

Perhaps the first thing to do is disable the ABS function. There’s a way to do it, pretty simple, might be just removing a fuse somewhere. Note that the pump has to keep working to provide power assist, as well as the rear brakes functioning at all. The idea is to disable just the electronic controls that release the brakes in the event of wheel lock. See if the problem reoccurs.

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The XJ40 and XJ-S had issues with the valve block discovered back in the 1990s.
The fault was the car jerking violently to one side upon brake application and at first Jaguar recommended replacing the complete brake actuator unit.

Then a TSB (70-27) was issued advising that an A.B.S. Valve Block repair kit is available to avoid the need to replace the complete brake actuator unit.

You can get the 'O’ring kit and just flush the valve block clean then reinstall. That will likely solve the issue.

TSB 70-27 January 1998

70-27 A.B.S. Valve Block Assembly.pdf (55.1 KB)

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I admit to knowing durn little if anything about ABS, much less Teves!!

But, Kirby’s comment on the sensors strike me as very logical given the symptoms.

I learned to live with the disparity in braking between my ABS equipped jeep and the non ABS Jaguar.

Carl

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Hmmm. I wonder if that repair kit is still available. And I wonder if that valve block can be disassembled and repaired.

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I think partial disassembly and cleaning (spraying with brake cleaner) will get the valves working again.
I have done this in the past.
The 'O’rings should be replaced but I don’t know if they are still available?

bob

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There are places to buy such O-rings. IIRC, you can buy them at a GM dealer. They are usually square cross section rings. Gotta be intended for brake systems; brake fluid is not compatible with normal nitrile O-rings. And I think the square cross section is necessary for holding the pressures involved when the O-ring is simply compressed between two housings rather than properly trapped within a fitting.

11

The valve block kit is NLA and it was about $1000 USD anyway.

I don’t remember if the 'O’rings were the square cut or regular round style?

I don’t think I have a spare old or damaged one in my ‘pile-o-parts’ to disassemble and check.

The XJ40 cars are similar but different part number.

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Kinda like these things: