Originally Posted by Peter J
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.



Thanks for that Peter very interesting article, it took me back to my metalwork class school days,when we produced metal castings in sand moulds......always very unnerving time handling the heavy very hot pouring ladle, trying to pour accurately, quickly with the least spitting and splash back................

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