Originally Posted by John V6
It is not the O rings. The brake piston housing is a porous casting


Correct:

This is an interesting read from the open university on the subject.
https://www.open.edu/openlearn/scie...ee-die-casting-active-atmosphere-casting

Clearly casting porosity is a known issue and some makers have a steel liner inside the casting.

Peter,
66, 2016 Porsche Boxster S
No longer driving Tarka, the 2014 Plus 8...