[s-cars] rear brake failure again... (long)

[Fred Munro]

James;

This type of failure is usually caused by the piston in the caliper failing
to retract completely after brake application, resulting in brake drag and
rapid pad & rotor wear. This in turn is usually caused by a sticky piston
(internal caliper corrosion resulting from failure to flush the fluid per
recommendations or contaminated fluid) or an obstructed brake flex line
(allows fluid past the obstruction on the high pressure pulse during brake
application but won't let the fluid drain back to retract the piston). Pads
that are seized in the carrier can also hang up in the "applied" position
and drag on the rotor.

If a pad is dragging, the rotor will be hot and you can smell burning pad
compound when you stop (BTDT).

On a side note, the pistons on the rear calipers have to be screwed back
into the caliper to retract them - they can't be simply pressed into the
caliper like the front brakes can. The e-brake mechanism on the rear brakes
"jacks" the piston out every time they are applied and keeps the pad
clearance correct.

You should replace your e-brake cable. Due to the equalizing bar on the
cables, you will have no e-brake if one is disconnected.

HTH

Fred Munro
'94 S4

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com]On Behalf Of James Bowes
Sent: December 3, 2004 10:15 AM
To: s-car-list at audifans.com
Subject: [s-cars] rear brake failure again... (long)


ok, here's an interesting one...

put on the BIRA system 3 brakes in May (1995.5 S6 avant), and soon
thereafter I found out that my rear brake on the drivers side was not only
worn out, but needed to be completely replaced. The diagnosis was that there
was a small amount of rust on the cylinder, thus making it impossible to
retract. Every time the brake was applied, it pushed it a little further out
with no way to retract. It machined away the whole rotor very quickly.

I replaced both rear rotors and both rear calipers with rebuilt ones. We
found that the parking brake cable was sticking on one side, so we cut the
cable on that side with plans to replace the whole cable later when I did my
exhaust.

Now I'm getting the same "ringing" noise from the same rear brake. I think I
might have the same problem as before, after only a few months. Haven't had
a chance to tear it all down yet, I'll do it this weekend and check.

I think my initial diagnosis about the rust on the caliper may have been a
red herring. Could it be that there is a different reason for this behavior
from the rear brake? Would a pinched SS line in the rear cause this?

Any advice would be helpful.. I'm going nuts here trying to figure out a way
to prevent a THIRD failure


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