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

[Konstantin Bogach]

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.