[s-cars] Brake Light and Thrumming Pedal

Fred Munro munrof at sympatico.ca
Fri Jun 30 20:19:08 EDT 2006


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.



-----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.


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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