[s-cars] Brake light stays on after start

Wed May 9 15:17:49 EDT 2007

Fred Munro wrote up a nice little procedure, that several have used with
great success.  This can be found on the S-Cars.org web site, in the
Download\PDF directory.

Download/PDF area:
http://www.s-cars.org/postnuke/modules.php?op=modload&name=Downloads&fil
e=index&req=viewdownload&cid=3

PDF:
http://s-cars.org/postnuke/downloads/pdf/Audi%20UrS4%20Central%20Hydraul
ic%20Fluid%20Flush.pdf

Unfortunately, the S4/S6 cars have a solid brake line from the brake
pressure booster to the return line to the reservoir from the PS rack.
Although the earlier 100/200s had an obnoxiou$ rubber/metal hose, it was
easier to disconnect that from the reservoir quickly to perform these
tests.  With our //S cars we need to remove the hard line prior to doing
this, inserting a substitute one that we can work with more quickly if
we want to run the tests prior to flushing the system.

This is Fred's test procedure from late June '06:

Douglas;

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 built 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

Hope this all helps... archived posts are good!

-S

Stott Hare

95 S6A

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] On Behalf Of Ivan Demkovitch
Sent: Wednesday, May 09, 2007 2:58 PM
To: s-car-list at audifans.com
Subject: [s-cars] Brake light stays on after start

I know it's been discussed before, just can't find anything. What is the
common resolution on this issue?

It seems that car has to work for about a minute for light to go out.

Bomb is good as it holds about 30 pumps. There is no problems with
brakes or anything. It's just this light. When I bought car few years
ago it was taking few seconds for it to go out. Now it could be like 1
minute. I wouldn't worry myself but potential buyer might. 

Light goes out fast if I start car soon after shutdown.

Thanks,

Ivan

93' S4

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