[s-cars] Brake Light and Thrumming Pedal

Fri Jun 30 14:59:54 EDT 2006

Brian,

What do you have to do to get to the bolts that hold the servo in
place?  I assume that the complete brake system needs a full bleed
afterward.  Is there a write up?

D.

On 6/30/06, brian hoeft <qweblog at gmail.com> wrote:
>
> sounds akin to my situation doug. the servo didnt leak profusely, and the
> pedal was stronger than aahnold in the eighties with the first pump after
> shutoff.
>
> soo,  the only other suggestion i have right now is to listen VERY carefully
> for an audible diesel-like sound..  i was able to hear it relatively easily
> in the cabin.  of course you should have your vents and radio off being as
> quiet as possible and just be very alert.
>
> remember, diesel-like-sound... :]   .. if i remember correctly the sound
> fluctuated a bit when touching the vibrating pedal.
>
> hopefully this is not the case and it is your accumulator.  much much easier
> to replace and muchmuchMUCH cheaper, or swap the emphasis because the servo
> was a borderline nightmare for me to replace.
>
> best wishes, //brian
>
>
>
> On 6/30/06, Douglas Fifield <douglas.fifield at gmail.com> wrote:
> > Fred and Everyone Else,
> >
> > Pushed the car out of the garage this morning, raised hood, had #1 son
> > start engine while I watched G002000 level in resivoir.  It was above
> > the full mark at about the seam level around the top of the tank.  It
> > did not move discernably the whole time the engine was running.  Brake
> > light was on and brake pedal thrumming.  Turned engine off and brake
> > pedal was rock hard.  Resivoir level never changed.
> >
> > What does this tell us?  I will try to find the materials to do the
> > servo leak test, but I don't have those materials readily available.
> >
> > D.
> >
> > On 6/29/06, Fred Munro < munrof at sympatico.ca> wrote:
> > > Douglas;
> > >
> > > Do the reservoir drawdown test I detailed in my earlier post. If the
> bomb is OK and the servo is leaking, the pump will partially charge the bomb
> and pull oil out of the reservoir with the engine running. Once you stop the
> engine, the oil will leak back out of the bomb through the servo and fill up
> the reservoir. If this happens, you'll have to test the servo to isolate it
> as the cause. This leak will be too large to do the Bentley servo test. You
> should use a test rig similar to what I described.
> > >
> > > If the bomb has failed, the pump will not put much oil into it and the
> level in the reservoir won't change much.
> > >
> > > Fred
> > >
> > >
> > >
> > > -----Original Message-----
> > > From: Douglas Fifield [mailto: douglas.fifield at gmail.com]
> > > Sent: June 29, 2006 4:24 PM
> > > To: Fred Munro
> > > Cc: s-car-list at audifans.com
> > > Subject: Re: [s-cars] Brake Light and Thrumming Pedal
> > >
> > >
> > > Fred et al,
> > >
> > > Things have progressed on the brake problem front.  Now, as soon as I
> > > turn off the engine, the brake pedal goes rock hard.
> > >
> > > I am not sure if this means the bomb has failed, or if the servo is
> > > leaking so badly that the bomb cannot be pressurized.
> > >
> > > Any thoughts?
> > >
> > > D.
> > >
> > >
> > > On 6/28/06, Fred Munro <munrof at sympatico.ca > wrote:
> > > > Douglas;
> > > >
> > > > The bomb has depressurized overnight. The brake light is on to warn of
> low
> > > > pressure in the brake boost system. The thrumming brake pedal is
> caused by
> > > > the pressure pulses from the positive displacement hydraulic pump.
> When you
> > > > apply the brakes with no stored pressure in the bomb, the boost
> pressure
> > > > comes from the pump and you feel the high and low pressure pulses as
> > > > vibration in the brake pedal.
> > > >
> > > > The bomb discharges due to a leaking check valve in the bomb or a
> leaking
> > > > brake servo. I used to believe the bomb check valve was the usual
> culprit,
> > > > but I now suspect the servo is usually at fault. The servo can leak by
> so
> > > > badly that the pump will never built up enough pressure to turn off
> the
> > > > warning light - I just had this problem in my '97 S6. If your warning
> light
> > > > doesn't go off, this is most likely your problem. A bad bomb check
> valve
> > > > will not prevent the bomb from pressurizing to design pressure.
> > > >
> > > > Flushing the central hydraulic fluid sometimes corrects the problem.
> Clean
> > > > the strainer in the reservoir when you flush the fluid.
> > > >
> > > > You can determine if you have a bad servo or check valve leak by
> running the
> > > > engine and noting the level in the reservoir. Shut off the engine and
> watch
> > > > the level. If you can see it visibly rising, either the servo is toast
> or
> > > > the bomb check valve is leaking badly. To test the servo, run the
> engine to
> > > > charge the bomb. Stop the engine. Remove the RETURN line on the servo
> (the
> > > > top connection). If more than a few drops of oil leaks out, the servo
> is
> > > > kaput. If you can see the fluid level rise in the reservoir, this test
> is
> > > > not a good one for two reasons:
> > > > 1. By the time you get the return line off, all the pressure has
> leaked out
> > > > and no oil will be released.
> > > > 2. If you do get the line off in time, so much oil will come out
> you'll have
> > > > it all over the rack and the driveway.
> > > >
> > > > I built a test rig to test my servo. I discharged the bomb completely
> by
> > > > pumping the brakes and removed the servo return line. I clamped off
> the
> > > > rubber section of the return line with a brake hose clamp. I bent a 8"
> > > > section of pre-made metric brake line at 90 degrees and threaded one
> end
> > > > into the return line port. I attached a plastic hose on the other end
> and
> > > > ran the hose into a jar. Start the engine (do NOT apply the brakes!!!)
> and
> > > > watch the plastic line. A flow of oil indicates a leaking servo. On
> the '97
> > > > the oil flow was a steady stream - the servo was completely shot.
> Funny, the
> > > > car had just been certified too. That explained why the warning light
> was
> > > > disconnected......
> > > >
> > > > HTH
> > > >
> > > > Fred Munro
> > > > '97 S6
> > > > '94 S4
> > > >
> > > > -----Original Message-----
> > > > From: s-car-list-bounces at audifans.com
> > > > [mailto: s-car-list-bounces at audifans.com]On Behalf Of Douglas Fifield
> > > > Sent: June 28, 2006 9:27 PM
> > > > To: s-car-list at audifans.com
> > > > Subject: [s-cars] Brake Light and Thrumming Pedal
> > > >
> > > >
> > > > Hedz,
> > > >
> > > > We have been over this one before, but I keep hoping that someone will
> > > > have figured it out and give me a clue.
> > > >
> > > > On and off, my S6 has had periods where the brake light stays on
> > > > beyond the normal 20 or so seconds in the morning.  These sessions
> > > > have been accompaning by a thrumming brake pedal and as long as that
> > > > continues, the brake light stays on.  In the past, the thrumming would
> > > > eventually stop and a few seconds later, the light would go out.
> > > > Today, it stayed with me and now the car is in the garage.
> > > >
> > > > It seems to stop normally.  Just had the brakes flushed last week and
> > > > the fluid levels are good.
> > > >
> > > > Any ideas?
> > > >
> > > > --
> > > > Douglas in MN
> > > > 95.5 Audi S//6 Avant
> > > > 73 BMW R60/5 mit Toaster Tank
> > > > _______________________________________________
> > > > S-CAR-List mailing list
> > > > S-CAR-List at audifans.com
> > > > http://www.audifans.com/mailman/listinfo/s-car-list
> > > >
> > > >
> > >
> > >
> > > --
> > > Douglas in MN
> > > 95.5 Audi S//6 Avant
> > > 73 BMW R60/5 mit Toaster Tank
> > >
> > >
> >
> >
> > --
> > Douglas in MN
> > 95.5 Audi S//6 Avant
> > 73 BMW R60/5 mit Toaster Tank
> > _______________________________________________
> > S-CAR-List mailing list
> > S-CAR-List at audifans.com
> > http://www.audifans.com/mailman/listinfo/s-car-list
> >
>
>

-- 
Douglas in MN
95.5 Audi S//6 Avant
73 BMW R60/5 mit Toaster Tank