[s-cars] Brake Light and Thrumming Pedal

[Douglas Fifield]

Never mind, Fred.  Cancel the newbie question posted earlier about
location.  Found it.

Did you have to replace the servo?

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
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>


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