[s-cars] Brake/Power steering system drama-

[Fred Munro]

Brian;

The steering and brake systems are independent to a certain extent but they
use a common fluid reservoir and pump. The pump has two circuits; a high
pressure low volume section for the brake assist and a low pressure high
volume section for the steering.

The light on and the hard brake pedal indicates you have low pressure in the
brake boost circuit and therefore little or no brake assist. The car is
dangerous to drive in this condition as it cannot be stopped easily without
brake assist (as I'm sure you've found out).

The system as installed is quite reliable when operating properly. It would
be much easier to sort out the hydraulic boost system than to cobble
together something else. Some of the older non-turbo cars (coupes, 4000's,
100) that you mention in your post use a vacuum brake boost system that
takes it's vacuum from the intake manifold. That would work fine until the
turbo kicked in and pressurized the manifold, hence the hydraulic system on
the turbo cars.

First of all, your "bomb" (hydraulic accumulator) is not holding pressure.
With the engine off, fluid is leaking out of the bomb and back into the
reservoir. You can confirm this by looking at the reservoir level with the
engine running and the light out and checking the level again after the car
sits overnight. If the level rises overnight, the bomb is leaking back into
the reservoir. This can be caused by a bad check valve in the bomb or a
leaking servo.

When you start up the car and get the light and a hard brake pedal, that
tells you that there is no stored pressure in the bomb to boost the brakes.
The fact that the light stays on for a long time and the pedal stays hard
tells me that the pump isn't pressurizing the circuit enough to boost the
brakes. Usually with a dead bomb and good servo the pump will build enough
pressure to give you easy braking for an "average" stop but not enough for a
panic stop. When you stop and give the pedal some "serious pushing" you are
pulsing the servo. Since this resolves the problem I suspect you have a
servo issue, probably hose debris clogging the internals. The servo is
probably leaking by so much that the pump can't pressurize the circuit.
After you pulse it, it stops leaking by and allows pressure to build in the
boost circuit. You can check this by doing a leak-by test on the servo.

I'd start by draining the hydraulic fluid, pulling all the banjo bolts to
clean the internal screens, and pulling and cleaning the screen in the
reservoir. Turn the wheels side to side (with the engine OFF) to force the
old oil out of the rack. Remove the reservoir and clean it out. Refill with
new oil and try it again. If this doesn't work and the servo fails the
leak-by test, you'll need a new servo. No problem, they only cost $800
*cough, cough*

HTH

Fred Munro
'97 S6 (mit new servo)

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com]On Behalf Of brian bilotti
Sent: November 19, 2007 12:45 AM
To: s-car-list at audifans.com
Subject: [s-cars] Brake/Power steering system drama-


S-Heads,
If you'll recall, awhile back I had the J hose blow.
Got it fixed at Pirtek, fast forward it blew again.
They fixed it.
Now on start up and for quite awhile the brake light
stays on and the brake pedal is extremely hard to
press. I end up pulling over and give the pedal some
serious pushing to force it down. Drive a little bit,
light goes off and everything's fine.
I'm really sick of this BS.....

Two questions:

1. Any ideas WTF this is?

2. Anybody change their brake system so the two
systems are independant of each other?

Javad has done it for the coupes and 4000 I think, but
he can't do it for the Urs. Another buddy of mine says
that a brake booster off a 100 and some other Audi
parts (my mind is blank on that one, maybe a master
cylinder) he seems to think this is a viable
project.... That said, I wanna ask if there's been any
BTDT out there.

Lemme know

Brian


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