[s-cars] occasional brake light on start and sometimes difficult steering

[Fred Munro]

Jared;

The hydraulic pump has two circuits; a high volume low pressure circuit for
the power steering and a low volume high pressure circuit for the brake
assist. Both circuits are fed from the common fluid reservoir. 

In the power steering circuit the pump pressurizes the fluid and sends it to
the steering rack. Fluid from the rack goes through an oil cooler (a
U-shaped black steel tube mounted at the bottom of the AC condenser) and
back to the reservoir.

In the brake boost circuit the pump pressurizes the fluid and sends it to
the brake boost servo and to the accumulator. The accumulator stores
pressurized fluid so you will have brake assist if the engine stops while
the car is in motion. Normally the pump pressurizes the accumulator until it
is charged and then fluid is bypassed to the reservoir. When you apply the
brakes, pressurized fluid flows into the servo to give brake assist and then
flows back into the reservoir. When the engine is stopped, the system should
hold pressure in the accumulator.

With age, the brake servo can leak by and/or the accumulator bypass valve
can leak by. This drains fluid from the accumulator back to the reservoir.
If enough fluid drains back, the pressure falls below the low pressure alarm
point and the "brake" light comes on to warn of low brake assist pressure.
This is the usual cause of the "brake light on in the morning, goes off 30
seconds after I start the car" syndrome. A sure sign of this happening is
the level in the reservoir rising overnight as the car sits.

The accumulator has nothing to do with the power steering circuit.

The steering problem is likely caused by low pressure in the steering
circuit or a sticky spool valve in the rack. The inside of the rubber hoses
start to break down with age and the little rubber particles clog the
screens in the banjo bolts and various orfices in the system. A flush of the
hydraulic system can eliminate the "light on in the morning" syndrome and
the loss of power steering assist by purging out the rubber particles. If
your fluid is black instead of clear green you need a flush.

One important note: these cars are IMPOSSIBLE to stop with no brake assist.
Your legs just aren't strong enough to generate effective manual braking
with this system. It is vitally important that the brake assist system be
kept in good working order.

HTH

Fred Munro
'97 S6


-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] On Behalf Of Jared Robinson
Sent: June 25, 2009 7:01 PM
To: s-car-list at audifans.com
Subject: [s-cars] occasional brake light on start and sometimes difficult
steering

So, I was reading some posts about the hydraulic system...
Once in a while (say once every 2-3 weeks) the brake light stays on for 30
seconds or less after starting the car. it normally goes away by the time I
rev the motor...
Also, sometimes when I'm at the end of my trip and pulling into my spot at
the train station, I go to turn the steering wheel from a stop and it's VERY
heavy... almost un-turnable... if I rev the motor, I get plenty of assist
though... It doesn't do it all the time though, sometimes it does it once a
week... sometimes twice.
I saw something about these two problems possibly being related...
It only really started doing it after I put 17" wheels on the car though. I
never noticed it with the 15" snow tires.

What's the story?

-- 
Jared Robinson
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