Brake Booster failure

have '95 xjs convert with only 42,000 miles. Had it out last week for a tour w/ the local British car club and at first just had intermittent power assist and eventually had no boost - just mechanical brakes. Can’t hear a vacuum leak. Does that mean the booster is shot or could it (hopefully) be something simple? Any help/advice appreciated.

Do you have Teves III or Teves IV? If you have a vacuum booster, it’s Teves IV. Teves III has an electric pump and a bulb-shaped pressure reservoir.

Just checked it out – is a bulb-shaped pressure reservoir.


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Teves III. That bulb itself is probably bad. The pump overheats and quits. Replace bulb w generic.

The Teves III is dangerous. A safety upgrade would be to rip it out and retrofit an earlier non-ABS vacuum boost brake system. Hard part is finding the pedal housing.

If the pump doesn’t work, the red and amber lights on the dash will be lit.
No pump function = no boost, Kirby is right.

Replacement accumulators (JLM1907) are hard to come by these days, expect to spend something on the order of $350-400$. The AC Delco equivalents that were on Amazon some years ago are long gone, no other “generic” parts available – there are cheaper accumulator spheres but they require adaptors and other modifications.

There is a small chance the switch is faulty (#3 on the diagram) but if the accumulator sphere has never been changed on 24 years, this is where I’d start.

Is this because of the belief that a single line failure will result in loss of pressure to the whole system? I recall that discussion but not the outcome, was it ever demonstrated? Or are there other issues with Teves III?

I had this information in the old photo albums, but they are lost, and probably a good idea to repost. May answer your question, Bob.

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Thanks so much for the advise and expertise - REALLY appreciate it!

Jim Champion

A couple of owners experienced it. Basically, it’s like this: The only pressure to the rear brakes is from the boost; no boost = no rear brakes. To get boost, you must apply force to the pedal to actuate the front brakes. If one of the flex hoses to a front brake blows and all the fluid squirts out, the pedal will fall to the floor without ever applying any pressure to the front brake circuit. If you can’t apply any pressure to the front brake circuit, you cannot call for any boost, so the rear brakes don’t get applied even though technically there’s nothing wrong with them. So, one hose failure results in the driver having to reach for the handbrake.

This seems different than the diagrams Steve posted describe. It says
“the lever mechanism moves the control valve forward, opening the port from the accumulator and closing the return port to the reservoir. Boost pressure is applied to the boost piston and rear brakes.”

It sounds like the mechanical connection between the actuating piston, the lever mechanism, and the control valve is what allows boost to be applied to the rear brakes, which seems independent of what is going on with the front brakes.

What am I missing?


It’s been a while, but as I recall the boost pressure is applied to the rear as you describe, but unless you can develop some resistance in the front circuit there will BE no boost pressure so the rears don’t engage.

That makes sense. Paraphrasing the above, “The Combined Pressure switch turns the pump on and off. The Pump is switched on and off as determined by system pressure…” You’d likely get some brief help from whatever is in the accumulator, but if a front brake line is ruptured and the pedal is low the volume that needs to be filled to produce boost is getting larger… while the driver continues to press down making it larger still.

System pressure and boost pressure are two different things. The pump provides system pressure, and it should always be available. But you don’t get boost pressure until force is applied to the brake pedal, and if it just falls to the floor you can’t apply any force to it.

Bob, please read “Dynamic” vs “Static” in the last paragraph. The diagram (color coded, also provided earlier by yours truly) and the description of what happens under normal operating conditions, front brakes are conventional brakes and the rear brakes are operated by the pressure supplied through the pump and stored in the accumulator.
The boost piston only increases the pedal force.

I tested my TEVES III brakes some years ago, having read on JL recommendations to remove the system altogether. IIRC, if you disconnect the power to the pump, you can see for yourself – the car will be stopping but the pedal will be much, much harder. Apparently, only the front two wheels will be doing the job.

Steve, this is great info. Still seems to me that Kirby has a valid point that this system could be in big trouble if a front brake line blows and dumps all pressure. But wouldn’t a blown brake line be big big trouble in any brake system?

Basic modern systems (like post 1968 on most American vehicles) are really two master cylinders in the same housing. The front and rear sections are independent. One can lose all the front fluid for whatever reason and still have rear brakes, and vice versa.

If my test shows that the car will stop with the electric pump disabled, i.e., no rear brakes whatsoever, doesn’t this mean that the Teves III also has two independent circuits?

Apologies to the intrusion in this thread and for asking naive questions – I am not an engineer and this is how I interpret the diagrams and my own empirical observations.

No worries at all.
The TevesIII does have two “sort of” independent circuits However, instead of two separate pistons (linked together so the pedal runs both) there is no rear piston to generate rear brake pressure. Instead the power assist or boost pump is used to pump pressure into the rear circuit. And it seems that ONLY happens when pressure is being created in the front circuit.

If all you lose is the electric pump, which means no rear brake pressure, and your front brake system is intact, you can stop with front brakes only. It will be difficult, because it’s front brakes only, AND you have no boost.

If you loose a front brake line, or for some other reason you lose front brake pressure, no front brakes. Since no front brake pressure, the pump doesn’t come on, so no rear brakes either.

Here is a typical dual-piston mastery cylinder:

You can imagine that the pistons are hypodermic plungers and are squeezing fluid into the brake lines. One line is front brakes, one is rear. You can see how the reservoirs re-fill the “hypodermic” as needed when the pistons retract.

If you look at the TevesIII drawings you’ll see that the rear piston is called the “boost” piston. It’s job is to push on the front piston. The reach chamber is pressurized by the pump, that pushes on the boost piston which pushes on the front piston, now you have power brakes. At the same time some of the boost pressure is routed out the bottom to the rear brakes. There is no piston squeezing fluid to the rear brakes. No pump and no boost pressure you can still push the front piston with the pedal, but it will be hard because the boost piston is not helping. And you’ll have no rear brakes.

Having said all that I am still not quite clear on what condition exactly switches on the pump. In the drawings it looks like the Combined Control Switch, which controls the pump, is independent of the front part of the system.


There are two circuits, but unfortunately they are not independent. The rear one will only function if the front circuit works. If NHTSA had figured out how it functioned, it’s a certainty it would have been banned in the US. Meanwhile Teves moved on to their Mk IV which doesn’t have the problem, actually has two independent brake circuits.