[s-cars] brake failure

[Brett Dikeman]

On Tue, Sep 22, 2009 at 10:48 PM, james McCarthy <jjssmccarthy at yahoo.com> wrote:
> Here's the story.
> Brakes have been running fine - thought have suspected the servo
> might be starting to go.

OK, what caused the suspicions?  This might shed light on the problem.

> Drive to work no problem.  Come out to drive home - no brakes - rock hard
> pedal.  Open hood and the p reservoir is is all wet on top - best I can figure it
> looks like the fluid was forced out of the cap.

That's because the system lost hydraulic power from the pump somehow,
and the bomb supplied all it could, releasing fluid into the system
loop from the bomb, and if your reservoir is at the max line, you'll
come extremely close to overflowing the tank, or overflow it slightly.
 That's why "max" is "maximum" and not "fill to", same as the dipstick
:-)

I assume the BRAKE light came on, right?  If not, that's odd, but
might be an important clue.

>  After 10 minutes of idling the brakes sort of came back - a bit firm but working. > So I limped home (3 miles) and immediately after stopping pedal was rock hard

Sure, this fits with not getting enough flow/pressure from the pump;
the bomb sloooowly recharged a little bit.  Question is: Why?  How's
the power steering?  If that's completely normal, than the belt is
fine.  The brakes are a different circuit in the pump, so if the pump
has failed on that side...

> Second - when first assembled the wheels were all locked -
> like the brakes were activated.  Separating the servo and
> MC released the wheels

So the car is undriveable (ie either assembled and wheels locked, or
sitting unassembled), or this issue was resolved?

> 2) Bomb?  it's less than two years old but possible I suppose...
> still doesn't explain the problem with the locked wheels after swapping the servo.

Doesn't sound like it, no.  I don't think the bomb failing would cause
the reservoir overflow.

I think the locked-wheels-problem is something separate, caused by the
"repair".

So, here's a test: pump the pedal until it is rock hard.  Now, have
someone watch the reservoir and start the car.  What happens to the
level?  Is the BRAKE light on or off?  The level should drop as the
system charges the bomb.  With a healthy system, this takes about 10
seconds tops at low idle, much quicker if you rev it up a bit.

Now, mark with a marker where the fluid level is.  Shut off the car.
Does it go back up?  Go get a sammich,  Come back.  Same level?  If
so, bomb and check valves are A-OK, and the problem is a clog (there
are small screens in most, if not all, of the banjo bolts...annoying.
And don't forget that all the crush washers are one-shot deals) or a
busted pump.

If the level has gone back up, either the servo is bleeding off
pressure, or the check valves have failed.

By the way, hydraulic fluid is a MUST CHANGE item on any Audi, given
how expensive all the components are.  It's cheap insurance to drain
the sucker every year or two and refill with fresh pentosin or the
near equivalent (and vastly cheaper) Mobil DTE-13M (which is not
appropriate for those of you who live where it gets way below freezing
or very toasty- it doesn't have as wide a temp range as synthetic
pentosin.)

-B