[s-cars] brake system problem

Fred Munro munrof at sympatico.ca
Thu May 12 19:46:52 EDT 2005


The S4 brake boost system consists of a hydraulic pump, a hydraulic pressure
accumulator (AKA the bomb), and a brake servo. The pump pressurizes the bomb
to 2000 psig or so. Depressing the brake pedal activates the brake servo
which uses stored hydraulic pressure in the bomb to stroke the master
cylinder and apply the brakes. The hydraulic pump recharges the bomb for the
next brake application. A good bomb will store enough pressurized fluid for
over 40 brake applications even if the engine is off (hydraulic pump not
running). If the pressure in the brake boost system is low, the brake
warning light will come on to warn you of a boost system failure.

It sounds like your bomb may be dead. The brake light is warning you the
boost pressure is low and the vibration you feel in the pedal are the
pressure pulses from the hydraulic pump as it tries to keep up with the
brake boost demand (it can't, the output volume is too small without the
reservoir provided by the "bomb").

Do the bomb test.

Start the engine and allow it to run until the brake warning light goes out.
Stop the engine.
Pump the brake pedal. Count the number of applications until the hydraulic
accumulator is exhausted (the brake pedal will get high and hard).
A good bomb will give over 40 applications.
If you get 10 or less replace the bomb.
The car is very dangerous to drive with a bad accumulator. If the boost
pressure is exhausted there is no way you will be able to apply enough pedal
pressure to stop the car. Trust me on this - you'll barely be able to stop
it rolling down a gentle grade in the driveway.


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 qbrian at ptd.net
Sent: May 11, 2005 11:50 PM
To: s-car-list at audifans.com
Subject: [s-cars] brake system problem

im hoping for help diagnosing, and a suggested repair if you think you know
the problem.

its a frighteningly fast vibrating,pulsating mess of a pedal travel. feels
as though the abs is actuating the whole way down.

the pads all around should be in good shape considering time. but the(e-
brake) brake warning light is now on.

interestingly enough, if i come to a complete stop, then press&hold the
pedal down with authority, the symptoms will clear temporarily.

also, theres an audible hiss the fist inch of pedal travel back upwards.

could i possibly have a leak that tainted the system with air or water?

i experienced similar in my 5000, but thanks to the nifty abs defeat switch
my s dissapointingly lacks, i was able to ignore the problem that
maraculously cured itself.

its said that timing is everything, and this is no exception. as i was
potentially driving to viginia so i could have both my 4k&urS at carlisle
next weekend.

say; sa-da-tay and feel okay...thanks for any help, -brian

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