[s-cars] 95 S6 Brake light

Sun Oct 19 07:35:56 PDT 2008

Rit, Dennis;

Any issues with the brake assist system on the UrS cars has to be taken VERY
seriously. It is literally not possible to stop these cars if the hydraulic
boost system fails completely. You have to experience this to really believe
it - I have experienced it just moving the car in the driveway with no brake
boost and it takes an inordinate amount of time and pedal pressure just to
stop the car from a 2 mph roll. A failure at highway speed? Pull the
handbrake and pray, because the main braking system will do nothing for you.
If you are driving with a dead bomb or a badly leaking servo, you are just a
broken serp belt or blown hydraulic pump/hose away from disaster.

Dennis, your issue may be a fussy level sensor in the reservoir but could
also be a servo issue. How many pedal pumps did you use to try to discharge
your bomb? It takes over 60 to discharge mine but the pedal does get rock
solid and high when it is discharged.
 Rit, your problem sounds like a bad servo - stroking the brake pedal slows
the leak-by and allows the system to pressurize the bomb. I've attached an
old post I wrote in response to a "brake light on in the morning" issue that
also describes the procedure I used to identify the bad servo on my '97 S6.
BTW, the servo costs a cool $800 or so but is relatively easy to install
(well, except for that one $@**$ bolt).

HTH

Fred Munro

>>ATTACHMENT<<

The bomb has depressurized overnight. The brake light is on to warn of low
pressure in the brake boost system. The thrumming brake pedal is caused by
the pressure pulses from the positive displacement hydraulic pump. When you
apply the brakes with no stored pressure in the bomb, the boost pressure
comes from the pump and you feel the high and low pressure pulses as
vibration in the brake pedal.

The bomb discharges due to a leaking check valve in the bomb or a leaking
brake servo. I used to believe the bomb check valve was the usual culprit,
but I now suspect the servo is usually at fault. The servo can leak by so
badly that the pump will never build up enough pressure to turn off the
warning light - I just had this problem in my '97 S6. If your warning light
doesn't go off, this is most likely your problem. A bad bomb check valve
will not prevent the bomb from pressurizing to design pressure.

Flushing the central hydraulic fluid sometimes corrects the problem. Clean
the strainer in the reservoir when you flush the fluid.

You can determine if you have a bad servo or check valve leak by running the
engine and noting the level in the reservoir. Shut off the engine and watch
the level. If you can see it visibly rising, either the servo is toast or
the bomb check valve is leaking badly. To test the servo, run the engine to
charge the bomb. Stop the engine. Remove the RETURN line on the servo (the
top connection). If more than a few drops of oil leaks out, the servo is
kaput. If you can see the fluid level rise in the reservoir, this test is
not a good one for two reasons:
1. By the time you get the return line off, all the pressure has leaked out
and no oil will be released.
2. If you do get the line off in time, so much oil will come out you'll have
it all over the rack and the driveway.

I built a test rig to test my servo. I discharged the bomb completely by
pumping the brakes and removed the servo return line. I clamped off the
rubber section of the return line with a brake hose clamp. I bent a 8"
section of pre-made metric brake line at 90 degrees and threaded one end
into the return line port. I attached a plastic hose on the other end and
ran the hose into a jar. Start the engine (do NOT apply the brakes!!!) and
watch the plastic line. A flow of oil indicates a leaking servo. On the '97
the oil flow was a steady stream - the servo was completely shot. Funny, the
car had just been certified too. That explained why the warning light was
disconnected......

HTH

Fred Munro
'97 S6
'94 S4

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com]On Behalf Of Rit Bellis
Sent: October 15, 2008 12:15 PM
To: 'Manuel Sanchez'; s-car-list at audifans.com
Cc: 'Mark Underwood'
Subject: Re: [s-cars] 95 S6 Brake light

My 93 s4 had similar red light issues before I replaced the bomb, and then
after when the hydraulic fluid was a tad low, the light would flicker on
slight grades. Once I filled the hydraulic reservoir properly, the issue
went away and did not reemerge for about 2 years.  Now, however, I have a
whole new issue: I start out with a rock hard pedal, then about thirty
seconds after starting, the pedal softens, and I can push it all the way to
the floor.  If I do this two or three times, the light goes out, pedal
pressure is restored to normal, and the light stays out till the next time I
shut the car off.  If I don't do this, the pedal acts like the bomb is dead,
the brake light stays on, I have virtually no brake assist for several
miles, and eventually (10 minutes or more) the brake light goes out and I
get assist back.  I have no observable fluid leaks in either the hydraulic
or the brake reservoirs.  It feels like the brake system has air in it, but
since hydraulic assist is utterly separate from brake fluid, I am flummoxed.
Any ideas, list?  It has been about 2 years since I replaced the bomb.

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] On Behalf Of Manuel Sanchez
Sent: Tuesday, October 14, 2008 7:23 PM
To: s-car-list at audifans.com
Cc: Mark Underwood
Subject: Re: [s-cars] 95 S6 Brake light

I thought the Brake Pad warning light was illuminated in the display
with the amber dots (can't think of the proper name of that display
at the moment), not the Red brake warning light?

I also have the little Red idiot light that stays on a bit longer in
the mornings, and I also get the the little Red light to flicker on
and off for no apparent reason at times when I am just driving down
the road. I don't have to be inducing any inputs (steering or
braking), just driving straight and steady and I'll get a little
flicker now and then, totally random. Sometimes there is a bump, but
sometimes the road is as smooth as glass (well not really).

I have no proof but I suspect the handbrake switch (I used to own a
Triumph TR-6, and I had this failure on that car, along with all
sorts of other Lucas electrical gremlins), just because it's on that
circuit I think.

I too have gone thru flushes of the Pentosin, 2 bombs, steering rack,
and brake pedal pumping that did not yield a stiff pedal for a long
while to no avail. When I returned my first replacement bomb I did
this because I was under the assumption that the bomb would fix the
morning start up check valve leak. It didn't in my case. I suppose I
could have received 2 defective replacement bombs, but I was thinking
the chances of that were unlikely, but who knows.

Dennis if you ever find out your problem, and I presume you are
talking about the Red light, please let me know. Sounds like we are
not alone.

Regards,

-manny

On Oct 14, 2008, at 3:00 PM, s-car-list-request at audifans.com wrote:

> Message: 4
> Date: Tue, 14 Oct 2008 11:16:36 -0600
> From: "Jerry Scott" <jerryscott at wispertel.net>
> Subject: Re: [s-cars] 95 S6 Brake light
> To: <dgraber460 at aol.com>
> Cc: s-car list <s-car-list at audifans.com>
> Message-ID: <8CDEBA1B0B874A339EFD9CD94204123D at Jerry5976>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> 	reply-type=original
>
> This could be the brake wear indicator.  If you cut off the
> original wires
> that go to the pad sensor, they need to be shorted at the
> connector.  Try
> unplugging the connector and cleaning the contacts, then
> reconnecting. Make
> sure that the shorted wires are making good contact.
> Jerry
> ----- Original Message -----
> From: <dgraber460 at aol.com>
> To: <s-car-list at audifans.com>
> Sent: Monday, October 13, 2008 8:27 PM
> Subject: [s-cars] 95 S6 Brake light
>
>
>> My 95 S6 brake light issue has been getting gradually worse for
>> about 3
>> years.
>> At first it would like many others?just not go out right away at
>> start up.
>> That has been getting progressively longer and longer and now it
>> is coming
>> on (off & on) during the drive.
>> I have flushed the fluid, replaced the bomb (new) and the pump.
>> Fluid is
>> above the line stopped and running.
>> If idling in the garage and turn the wheel left to right the light
>> will
>> comes on and go off. Use the brakes it comes on and goes off. Hit
>> a bump
>> it comes on and goes off
>> The brakes have never given me any obvious reason to be concerned
>> about
>> failure as they have felt the same through all of this. They don't
>> feel
>> crisp but not as if they cold fail. The usual bomb test of pedal
>> pumping
>> with the car off does not give a rock hard pedal after any number of
>> pumps. Never did.
>> Pads are all good and rotors are normal for their miles.
>> Equipped with G60s & drilled rotors. I would like it to feel sharp
>> like
>> the URQ with the same equipment.
>> Any ideas? TIA
>>

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