1996 Cabriolet Hydraulic Servo fault

[mhobson]

Hello,

My daughter has a 1996 2.6 cab, and has had an intermittent brake fault in that the brake servo sometimes does not work, especially just after starting the engine but sometimes when she is driving, which I find very worrying, so I took it to a local dealer who says that the hydraulic pressure is too low on the system. I have no dealings with hydraulic servos before only vacuum servos. I understand that the pressure is generated by the power steering pump and stored in an accumulator. The garage I took it to is in France where I live, my daughter is here on holiday, and the mechanic says it needs a new 'boitier hydraulic' which translated I think means hydraulic gearbox and although I have not got a quote yet, he says would be as much or more than the car is worth.

I would appreciate any advice and suggestions as to how I may be able to solve the problem, are second hand parts likely to be available. The power steering seems to be fine so I cannot understand why the pressure in the brakes is low, does this mean that it needs a new hydraulic accumulator, and if so how difficult would it be to change?

I hope to hear from someone soon,

Regards to all,


Michael

[scotty33]

It is an unusual system, but the power steering pump is actually 2 separate pumps, one for the steering, the other feeds the hydraulic brake assist system. The brake assist system has only, the pump, servo and a pressure accumulator - probably the 'boitier' your mech refers to?
The accumulator often fails and can be tested as in the 20v link below, but also test the servo itself, to rule it out, before buying an accumulator. Beware there may be a great deal of pressure in the servo 'feed' hose, it is the servo 'return' hose that needs to be tested!

Brake & Hydraulic Problems (http://20v.org/brakprob.htm)

and you may be interested in a new accumulator:

parts for type B3 AND B4* (80/90/S2/RS2) (http://homepage.internet.lu/customautocraft/typeB3-B4_80_90_S2_RS2.htm#BRAKES)

You must use hydraulic oil, ideally to Audi spec G002000 or the newer standard G004000. Normal PAS fluid causes leaks everywhere, and these parts get expensive..

[mhobson]

Thank you Scotty,

I did not have the kit to test the pressure but the VW/Audi man did. He says it is definitely the accumulator but is suspicious about the servo as well. I had hoped to try changing the accumulator to begin with and then re-evaluate. I did contact Paulo in Luxembourg and a more helpful man it would be hard to find but he does not have any accumulators in stock and may not get any more in this year. I have contacted a firm in Switzerlad, TOT spares, but have had no reply, they may be away on holiday. There is a rather rusty looking s/h one for sale on Ebay UK, and my son says that there is a coupe in a scrap yard in cornwall which he hopes to look at for me. The price of the parts alone to repair the car in France by Audi is around 1200 Euros, which is maybe more than the car is worth.

If the fault with the accumulator is a perforated diaphragm, it will of course have to be changed, but I have heard of some tales of dirty hydraulic fluid and wonder if dirt particles got into the seat of the pressure valve, it mat allow some pressure to bypass. I thought about changing the fluid, which I realise must be depressurised first but I do not know how I should try to bleed the section of pipe between the accumulator and the master cylinder. I wonder why the problem is so inermittent, mostly the brakes work fine.

Regards,

Michael

[scotty33]

It is easy to depressurise the brake system, just do the 'bomb' test from the 20v link, when the pedal goes 'hard' there is no more pressure stored in the accumulator ( if there was any in the first place!)
I can see that degraded fluid could contain debris, which may hold the relief valve from seating, or cause a bypass within the servo. Either situation would cause the system to lose pressure and would therefore struggle to maintain pressure. It would be a good idea to flush out the entire brake and hyd assist systems just to rule this out, If you will be fitting new parts anyway in due cause it would be madness not to do it at that point. If this were the case I would expect the fluid level in the reservoir to drop after the engine starts, there would be some fluid stored in the accumulator, but it would quite quickly find it's way back to the res when you shut off the engine.
If the diaphragm in the servo is failed, I would not expect the level to change?
The usual problem, is that these accumulators are supposed to hold enough fluid for the driver to stop safely when the engine stops turning. If you press the brakes once and all is ok, then press them again straight away, this is a good test as to whether there is a real problem, and could be why the symptom 'seems' intermittent.
There is no kit required to test the pressure, if the autocheck warning appears at any point, the system pressure is low at that time (set to 116 bar iirc).
It could also be the pump failing, but probably less likely.

To clean the system:

http://www.sjmautotechnik.com/trouble_shooting/susp.html#hydoil

[vanhow]

Hello there
I have what may be a similar problem. I also have a 1996, 2.6 cabriolet and recently the brake pedal has started to "stick" or "shudder". The disks and pads are new - so it seems like an hydraulic problem. I have just had the pump replaced (new) at great expense, but the problem persists. When depressing the brake pedal, the pedal sticks/shudders intermittently. The same applies when releasing the pedal. (With the engine running, this happens irrespective of whether the car is moving or not). For the benefit of others who may have a similar problem - don't replace the pump as this has not cured the problem. Any suggestions would be greatly appreciated.


Rgds

[mhobson]

Hello,

First of all the fine advice given by Scotty should be followed to check the problem.

I am getting my daughters accumulator changed today.

If you think the accumulator is at fault there are plenty of second hand accumulators for sale but seeing as they have a life of only about 7 years, unless you are sure of how old they are I would not buy anything except new and the Audi price quoted by the garage was astronomical. I did a great deal of searching and the only supplier in Europe who I could find with stock was in Germany on German ebay and the item is listed as:
Druckspeicher Bremsanlage passend für Audi 80 B4 Coupe FABRIKNEUThe item number was: 360446862995

The price I paid was about 244 Euros including carriage (DHL)

According to his listing he still has three left, apparently there are only a few available each year.

Hope this helps,

Regards, Michael

[vanhow]

Thanks Michael.

[mhobson]

Got the car back today from the VW garage who fitted the accumulator.

He showed metal fragments in the hydraulic (not brake) fluid and suspects that there is another problem in the system which may be causing this and suspects the brake servo.

I have no experience in hydraulic brake assistance only vacuum servos.

I would be grateful to consider suggestions as to what the problem may be.

Regards,


Michael

[scotty33]

The tandem (power steering) pump and maybe the steering rack, could produce metal particles, I guess they would anyway, over time, which is why I think it is a good idea to change fluid at a frequency. I think the whole system would be at more risk, if the reservoir is topped up with PAS fluid rather than the correct hydraulic oil. Even the older G002000 fluid is mineral based which will degrade over time and lead to reduced protection from wear. The newer G004000 is synthetic based but is still not going to last forever. I really think the cost of the pump/servo/rack warrants a fluid change.
btw it is also possible to revert to a vacuum servo and conventional pas pump, the 2.3 10v 90 and coupe use this system.
It is also possible to repressurise the nitrogen precharge in the accumulator. This should solve the problem if the diaphragm inside is intact. I guess a hydraulic engineers could do this, if they have the right adaptor to connect the nitrogen bottle. needs to be done with the accumulator empty, so probably easier if done off the car? Maybe worth a try before splashing out on a new accumulator, I did not realise they are soo expensive!
I hope the mechanic flushed out the entire brake assist and pas circuits before fitting the new accumulator? If not he will have contaminated the new part with debris..

[mhobson]

Thank you Scotty, yet again,

Well the new accumulator has been fitted and the hydraulic fluid has been changed as you recommended, and I would have insisted on anyway and these particles were in the old fluid, as it was a VW garage I expect the correct fluid was used, most likely the older spec i suspect. (The garages always seem to use the original spec on parts even when new improved parts become available, the oil mist separator for the 2.5 V6 comes to mind as they insisted on supplying my earlier A6 with the original dreadful nylon fabric design rather than the newer centrifugal design filter which cannot get blocked because it was not listed as original equipment).

Pleased to say with the new accumulator fitted the brake warning light has not come on again and the brake assist so far has been functioning correctly, so it seems it needed changing as your test sequence highlighted.

By the way the man in Luxembourg, Paul I think, was extremely helpful but he did not expect to get any more accumulators in for the rest of the year.

Very best regards,


Michael