Brake Booster failure

That’s the exact plan. Just renewed the handbrake pads and will adjust properly. I may test on jack stands first. I’ll video and post. Will be interesting.

We are all still waiting with baited breath for the results of your test with a front brake line disconnected!!!

Well… not gonna happen. :). I converted back to vac booster.

Bob Brackney

Ok, Thanks for the reply Bob.

Alan Erickson

IMG_6144.jpg

Surely difficult and made in China only. But hey, that’s the reason for this forum. Why not swapping it from one fitted f.eg. in Maybach?

Tested, working finest. Believe it or not - it was £80ish on Amazon with Prime next day delivery

I have a Lemfoerder accumulator on mine (have to look up the part number), which I found on a hot-rod website, but your idea to use the Mercedes part seems to be a better deal.

For posterity, Bosch 0265202070 is MB 0004302694 (cross-reference to 0004301394
and 0004302394)

Fits 2003-06. E320, e320 cdi, e350, e500. Sl500, sl600, sl55 amg. Sl65 amg. Sl-class. E55 amg. Sl550. Sl63 amg.

Good source for it in the US (besides Amazon):

Thanks for posting!

:-((

I needed that test Bob, for sanity check.

This topic came up again (not surprisingly) and led to a heated discussion. Here:

Ok, Steve,

Let’s keep the ball rolling, actual test vs tribe knowledge.

As I’m planning to replace all the hydraulic hoses anyway, I will sacrifice my over-priced time and part of social benefit income to check the scenario with no front circuit pressure.
The only bits that can be messed up are the piston seals due to excessive diving.
But hey, it’s science…

If that will satisfy the public - I’ll use half-measure:
Stationary test with pressure gauge at the back and front disconnected from one side only.

Kirbert,
You’re free to add your bits as testing criteria may be insufficient…

1 Like

Where did you get the change over vacuum system as I am looking at doing the same.?

Hello-
All the details are here:

Well, note that that particular operation turned into a debacle due to the need for a clutch pedal and the owner’s decision to make the Teves III pedal box work. If you don’t need a clutch pedal, the conversion is 100X easier, just find a pre-1988 pedal box from an XJ-S and everything drops in. Even if you do need a clutch pedal, I’d argue there are easier ways to go about it.

Hi All, old thread revival here.

I was looking for information regarding cars fitted with Teves IV systems and came cross the “other” thread discussing making parts for Teves III, which led to some disagreement about it’s safety…which then led me here.

I don’t have any knowledge or experience with Jaguar power brakes, but do have some knowledge of Mercedes SBC and extensive experience of Citroen power brakes.

Having read through the information posted earlier in this thread I believe both XjsBanger and Kirbert are partially correct, but not strictly for the correct reasons.

The Jaguar/Teves III is a front/rear split system:
In static mode (normal braking, no ABS in use), the front brakes only are fed from the reservoir by gravity.
The rear brakes are always fed by pressurised fluid from the accumulator.

When the driver demands brakes the control valve admits fluid into the rear section of the hydraulic actuator, which directly applies the rear brakes and causes pressure to build behind the boost piston.
There is no rear brake piston, the rear brakes are applied as a direct result of the control valve admitting fluid.
The drivers leg and the pressure behind the boost piston pushes the master piston forwards so the central valve closes the return to the reservoir, thus as the master piston advances further it pressurises the gravity filled front brakes master cylinder and pipework.

If the power brake high pressure supply fails, such as failed pump, the warning light will illuminate and the driver will have unassisted front brakes only, due to being gravity fed from the reservoir.

Where the Jaguar-Teves III system is complicated is when ABS is in use.
To feed the volume of pressurised fluid required for the recirculating ABS system, both front and rear brakes are fed from the reserve of pressurised fluid.
Instead of the front master cylinder being gravity fed and the pressure amplification being provided by the boost piston pressing the sealed volume of fluid forwards…central valve closed: both the main and central valves open and the necessary fluid volume is now all provided by the pre-pressurised feed.
When ABS is in operation the master piston has no bearing on brake fluid pressure and flow, that is controlled soley by the control valve.

Theoretically, in dynamic operation (Abs in use), if a front brake pipe was to rupture fluid pressure in the front master cylinder would be depleted, the main and central valves would be open and fluid pressure would be depleted in the entire brake booster unit and eventually/pretty quickly, the reservoir would run out of fluid.

When in static mode (no ABS operation), the main valve will be closed so there will be no transfer of fluid from the rear of the main hydraulic unit to the front brake master cylinder: the front brakes would be lost, but rears would still be functional.

In essence, if the ABS system applies power to the main valve you are doomed, but as long as that solenoid valve is closed and the high pressure system is functioning, you should live to fight another day.
In reality if a front brake pipe failed I suspect the wheels unlocking due to lost pressure, would cancel the ABS trigger, so the system would close the main valve, thus restoring pressure in the rear brakes, assuming there was still fluid in the reservoir.
As long as the high pressure system is in good order, with regularly changed fluid to ensure the main valve isn’t seized, then the system should retain functionality.
If the main valve is in a poor condition allowing it to jam in the open position, you could well have a problem.
Say the valve, which is default closed, had after some years been opened due to an ABS operation and then stuck open, both brake circuits would be being operated by the high pressure fluid.
If at some later stage a front brake pipe failed, the system would indeed fail.
This is assuming there is no error reporting for the main valve position.

A simple check for a stuck main valve would be to open a front brake bleed nipple and press and hold the brake pedal down.
If the front brakes are, as they should be, being gravity fed the flow of fluid will stop, if being high pressure fed it will continue.

For their SBC system Mercedes employ an additional, completely separate, un-assisted, gravity fed front brake circuit as their failsafe and Citroen use two completely separate systems for their front and rear braking circuits, each supplied from independent sources: front being the high pressure pump, rear being the rear suspension.

All manufacturers would like to have high pressure braking systems, but patents hinder that.

If I had a car with Teves III system I wouldn’t change it, but would ensure it is serviced and the main valve is operational.
If I was to change to a servo system I would try to use a Mercedes unit from a W210 “E class” fitted with BAS. Two servos stacked together where only one is used in normal braking, both used for emergency braking, dictated by the speed of reaction of pressing the brake pedal.

1 Like

So which MYs did Jag use the Teves III system and which the Teves IV system? My '94 4.0 has a _____ ? :confused: Also, from what is being said here (esp. from Palmdude), it sounds like this is one system where Jag did NOT make improvements to the system as time went by, but actually made “disimprovements” … (?) :roll_eyes:

1 Like

TEVES IV was only in the 1995.25 and later XJS, both 4.0 and 6.0. Much simpler system.

They improved it by going to version IV. And it’s not really Jag, sort of, they weren’t the only ones to use that system. Anywhere you can find it was used, you can find people online lamenting the fact. I’m sure it seemed like a good idea at the time.

I didn’t see that answered?

I believe from 89 to 95.25.

89 till right before the end when they went to the newer system, Google a couple photos of cars around 93 and up and see where they start having a brake booster again instead of the giant plastic fluid tank.

95.25 was the phase in of Teves IV.

Correct. The Teves Mk IV-GI system is listed as a tech update for the 1995.25 models. No accumulator ball.