[s-cars] Brake light staying on after start up

[Fred Munro]

Hi Tim;

This is a common issue with the UrS cars. These cars use pressurized
hydraulic fluid to provide brake boost; Audi did not use a vacuum boost
system due to the complications imposed by the turbocharger. The light is on
because the hydraulic pressure in the brake boost system is low. The
hydraulic accumulator (AKA "the bomb") is charged by the hydraulic pump and
stores hydraulic fluid at ~3000 psig. If your engine quits or heavy braking
demands more fluid for boost than the pump can put out, the fluid for brake
boost is supplied from the bomb. If pressure in the boost system gets low,
the red brake warning light comes on.
The light comes on when the car is started after sitting for a while because
the hydraulic pressure in the system has dropped. The most likely causes
are:
1. Leaking check valve in the bomb allowing fluid to flow from the bomb back
into the reservoir.
2. Leaking brake boost servo allowing fluid to flow from the bomb into the
reservoir.
3. Bad bomb with reduced reserve capacity.

10 seconds isn't bad; some listers wait 30 - 40 seconds for the light to go
off.

Flushing the hydraulic fluid will sometimes fix this problem.

***NOTE*** These cars CANNOT be stopped without brake boost. If you suspect
a problem with bomb capacity, do not drive the car. It's all you can do to
stop them just moving them around in the driveway if you have no boost.


HTH

Fred Munro
'94 S4



-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com]On Behalf Of Premiere Investment
Properties, LLC.
Sent: October 23, 2005 7:05 AM
To: audifans
Subject: [s-cars] Brake light staying on after start up


Hi all
I just bought a 92 S4 last week and am trying to work out the bugs on the
car...The brake light(the little red one, when you pull the e-brake) will
stay on for 10 second or so before it will shut off. What could be the cause
of this. Its been awhile since I've owned an audi and still a little rusty
Thanks for any help
Tim

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