[s-cars] Brake Light and Thrumming Pedal

Fri Jun 30 20:58:00 EDT 2006

woops doug my mistake, you dont need to bleed.  my master cyl was original
with ?180k, i got a nice deal, i replaced it at the same time,, sooo, i
bled.  though while doing it, i remember it not passing by the lines without
a gentle aside, so just be gentle.

//brian

On 6/30/06, Fred Munro <munrof at sympatico.ca> wrote:
>
> Douglas;
>
> If the level doesn't fall, the bomb is not being charged with fluid. This
> could be a really bad servo or a bad pump. At this point I'd do the servo
> test to rule that out. A totally dead bomb would also not draw fluid out of
> the reservoir, but it should allow the system to pressurize and the light
> would go out.
>
> I replaced the servo in a couple hours. You don't need to do a brake bleed
> - you remove the two bolts securing the master cylinder to the servo,
> release the brake lines to the MC from the multi-line clamp, and slide the
> MC off the servo without removing the brake lines. When the new servo is
> installed, you slide the MC back onto the servo and bolt it up. You don't
> have to open up the lines or bleed the brakes.
>
> As Brian mentioned, the servo is secured by four 13mm nuts that also
> secure the pedal cluster. The top left nut is a bit tricky as Brian points
> out, but I got it using 18" of 1/4" drive extensions and a flex end. You
> have to use 1/4" drive stuff or you can't see past the extensions to the
> nut. You have to use multiple extensions and assemble them as you insert
> them through the hole in the pedal cluster. Remember to get a new sponge
> gasket to seal the servo to the firewall. Unfortunately I didn't do a
> write-up on this.
>
> Hopefully you don't need a servo, but the alternative is probably the
> pump. If the servo tests good, you'll have to make up a pressure testing rig
> or take the car to the dealer to test the pump output pressure.
>
> HTH
>
> Fred
>
> -----Original Message-----
> From: Douglas Fifield [mailto:douglas.fifield at gmail.com]
> Sent: June 30, 2006 11:07 AM
> To: Fred Munro
> Cc: s-car-list at audifans.com
> Subject: Re: [s-cars] Brake Light and Thrumming Pedal
>
>
> 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
>
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