A new form of e-brake failure

[Kneale Brownson]

Paul,  from looking at the piston in the '86 5Ktq rear caliper I took 
apart, it appears that the  captive "nut" inside the piston is somehow 
pressed in place on the older design and efforts to turn the piston when 
the threaded rod in the cylinder and/or the "nut" are corroded result in 
the "nut" and captivating assembly somehow detaching from the inside of the 
piston.  In the newer cars (like my '91 200), the part that's threaded onto 
the rod in the cylinder is held in place with an internal snapring.   I'd 
guess your piston has reached the failed status.

BTW, on your website write-up of rear brake caliper disassembly, you 
suggest removing the threaded rod by applying a nut to provide a perch for 
levering the rod out.  The threads on this rod are not any sort of standard 
metric thread.  I took it to a thorough metric fastener supply house and 
they could find no commercially-available nut to match that thread.  I've 
extracted the rod by persistent levering from two sides at once with  heavy 
screwdrivers and sort of alternating a bit of extra pressure from side to 
side.  The far end of the rod is machined quite closely to fit into the 
cavity for it.


At 02:11 PM 08/28/2001 +0100, Paul Heneghan wrote:

>Hello everyone,
>
>I've got a new type of handbrake problem (car is a 1984 Audi 80 quattro -
>similar to a 4KSQ, rear brakes also similar to Audi 100 or 5K) which I've
>never seen before and which doesn't appear to be documented in the archives.
>
>I'm familiar with all the usual forms of problems, usually resulting in no
>handbrake operation at the caliper:
>seized handbrake cables
>rusted parking brake mechanism operating shafts
>rusted threaded shaft piston
>
>I've even plagiarised the works of others, added my own caliper rebuild
>experiences and published a fairly lengthy web page on this topic at
>http://heneghan.members.beeb.net/audi/parkingbrake.html
>
>The new problem is a self-adjusting mechanism that apparently allows the
>piston to retract when the handbrake is put on.  The handbrake mech. should
>press on a lozenge-shaped rod (about 15mm long) which in turn should press
>on a small piston at the end of the very large threaded shaft.  This screws
>into the piston which pushes the pads together.  There is a self-adjusting
>mech. in the piston which allows it to very slowly unscrew itself (the
>external part of the piston doesn't need to rotate, only the internal part)
>outwards to compensate for pad wear.  In my faulty caliper, it is possible
>to press on the piston and push it back into the caliper without needing to
>rotate it which implies a collapsing or reversing self-adjusting mechanism.
>This cannot be right, can it?
>
>Any help gratefully received.
>
>Paul
>
>
>
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