200TQ brake lock

[Bernard Littau]

Hi Tom,

Different kind of stiff.  In your case (bernard reaches into hat of Audi 
problems and pull out slip of paper, reading:) sounds more like a bad 
bomb and/or hydraulic pump.

In Chris's case, the key differential in diagnosis was the driving in 
stop-and-go traffic and brake calipers locking up.  The stiffness of the 
pedal was kind of secondary to that.  What happens in this case is the 
brake master cylinder somehow warps due to high heat and age, preventing 
the plunger from fully retracting and venting brake fluid from the line 
into the reservoir.  This, coupled with heat on the calipers, causes the 
now not venting brake fluid to expand and locks up the calipers, which 
causes more heat, which makes the calipers more tight...   The pedal 
becomes tight because you are pushing against the warpage and the 
accumulated unvented brake fluid pressure.

In your case, from a cold start, the hard brake pedal is much more 
likely to be due to a lack of power assist on the brakes.  Normally, you 
should have the brake failure warning come on in your autocheck in such 
a situation.  Check if the wires from the brake booster pressure sensor 
have been disconnected.  The hydraulic brake booster is between the 
brake master cylinder and the firewall.  The sensor is on the low part 
of the booster.

Best,

Bernard

tom winter wrote:

>J wrote: My brake pedal gets completely stiff
>
>Bernard replied: Bad brake master cylinder. Replace it ASAP.
>
>I'm thinking that my '89 100q sedan might have the same problem. It was
>parked for a month or so, and when I just drove it yesterday the brake pedal
>felt really stiff and "boardy". I stomped on it and it came back up, not all
>the way, though, still 1 cm left. It felt better after it was parked, but
>was still really hard and boardy when I started it up again. I'm not driving
>it right now, but I will want to soon.
>
>Same problem? Brake Master Cylinder? If so, any advice to me before I start
>tearing the car apart to replace it?
>
>Thanks in advance.
>
>Tom
>
>
>
>  
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