[s-cars] Brake Light and Thrumming Pedal

[Fred Munro]

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

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com]On Behalf Of Douglas Fifield
Sent: June 28, 2006 9:27 PM
To: s-car-list at audifans.com
Subject: [s-cars] Brake Light and Thrumming Pedal


Hedz,

We have been over this one before, but I keep hoping that someone will
have figured it out and give me a clue.

On and off, my S6 has had periods where the brake light stays on
beyond the normal 20 or so seconds in the morning.  These sessions
have been accompaning by a thrumming brake pedal and as long as that
continues, the brake light stays on.  In the past, the thrumming would
eventually stop and a few seconds later, the light would go out.
Today, it stayed with me and now the car is in the garage.

It seems to stop normally.  Just had the brakes flushed last week and
the fluid levels are good.

Any ideas?

--
Douglas in MN
95.5 Audi S//6 Avant
73 BMW R60/5 mit Toaster Tank
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