surprise: o-ring and dirt inside of braks piston.

[james accordino]

I don't understand the "bearing" part.  You have the
dust boot at the top.  One side is in the caliper body
and the other in the top ring machined in the piston.
The next groove down on the piston is for a square cut
o-ring.  This seals brake fluid pressure behind the
piston and drives it out.  The only snap ring I found
was keeping the strangely threaded rod that holds the
piston into the bottom of the caliper.  There's
another o-ring down there that keeps the brake fluid
out of the e-brake cam chamber.  Did you disassemble
the stuff inside the piston?  I did this on my first
"test" one and found that this wasn't the problem and
probably didn't need to be disassembled.  I might have
done it to clean it on some of them, but it wasn't the
problem IME.  The dust boot and square cut o-ring come
in a caliper "rebuild kit".  The one's I've got in
stock are Lucas parts.  It says REP.-SATZ, then I
think this is the Lucas # SP 7381.  Then its got an
Audi # on another sticker. 431 698 471, with a capital
B underneath.  I "think" these are for the 38mm rear
caliper like on the 200.  2 dust boots and 2 square
cut o-rings.  I think I remember them being like $6 or
$8 per box.  Sorry I can't be more vague, but my
inventory and billing systems suck and the secretary
isn't in.

HTH
Jim Accordino

--- Konstantin Bogach <kbogach at comcast.net> wrote:
> Having,  too much probably,  free time I removed
> circlip from rear
> caliper piston and found another o-ring there and a
> cavity between it
> and the top of the piston filled with dirt.  Then I
> found a small hole
> on the side of piston, in the groove for rubber
> boot, which goes into
> the cavity.  Plus, it is a bearing under circlip.
> Can some explain the
> purpose of this complex structure. Why cavity
> (termal insulation?), why
> bearing?  Anybody know what kind of o-ring is that?
> It does not look
> like regular o-ring.  I am afraid to stratch it if I
> pull it off.
>
> Thanks.
>
> Konstantin Bogach.
>


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