Typ 89 leaking rear caliper - source for o-rings?

[Radek]

Hello Group.

The rear brake caliper on my 88 90Q is leaking along the handbrake shaft.
Would anyone have a source for the internal o-ring that's probably the
culprit?  I had the same thing happen last winter and could not find a
replacement o-ring.  I understand it's a special kind of rubber, brake fluid
resistant.  I disassembled everything, cleaned and put back together.  The
leak was gone for ten months but now it's back (perhaps related to the cold
weather???)?  Has anyone experienced this and found a solution?
TIA.

Radek
88 90Q
91 V8Q 5-sp.