[s-cars] soft brake pedal

[Tom Green]

You are definitely barking up the wrong tree if you think you are  
going to remove the brake booster
and shake it to check for loose parts.  The first link shows the  
hydraulic brake booster in the UrS4.
The second link has a photo of a vacuum brake booster similar to that  
in the Quantum Syncro.  I
suggest visiting quattroworld.com for the UrS4/UrS6/S2/RS2 forum for  
the FAQ at the top of the forum
page.  There should be sufficient information there to get you going  
and some known good suppliers
for parts.  Hopefully you won't need that expensive booster but can  
probably find a used or even
rebuilt  part.

The most common failure for the booster is for it to leak hyd pressure  
past internal seals back to the
reservoir, which will bleed off pressure in the system after shutdown  
and cause the pump to not be able
to keep up with demand during operation.  You can remove the return  
line from the servo with the engine
off but pressure still in the system to see if it leaks pressure.  If  
the fluid is dirty, it is usually dark from hose
bits eroded over time.  A fluid flush and filter cleaning may disloge  
some bits that are obstructing proper
valve operation or restricting fluid flow and pressure.  A magnefine  
filter in the return line can help keep
the system clean.


On Monday December 19, 2011, at 10:57 AM, Nicholas <applen at gmail.com>  
wrote:

> So now my brake warning light hardly ever comes on (not sure what to
> make of that) and bleeding the brakes didn't seem to get rid of my
> pedal-travel issue. And I'd be surprised if that had anything to do
> with the intermittent warning light.
>
> On the syncronized forum (I've got a Quantum Syncro Wagon too),
> someone just mentioned that they are having the same issue and
> replaced pretty much everything in the brake system before replacing
> the brake booster. Turns out their booster had corroded on the inside
> and very little was left of the diaphragm.
>
> So, could I just have a worn-out brake booster? I plan on pulling mine
> and shaking it so see if it feel like anything is rotten or loose in
> there (a preliminary test for sure).
>
> Searching for possible replacements I found a wide, wide range of
> prices and images of what the booster actually is:
>
> http://www.partsgeek.com/ymm/1992/audi/s4.html?category=brake&part=brake_booster
>
> http://www.theautopartsshop.com/power-brake-booster/cd532671.html
>
> ...which one is (more) right?
>
> Or, am I barking up the wrong tree here?
>
> Cheers,
> -Nick