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What are 'ceramic type' inserts in the pistons for?
Mine are crumbling and I cannot find any replacement inserts or complete with piston housing, are they needed as they stand slightly proud of the end of the metal part of the piston?

its not an insert per se, but a physical part of the piston.

you have two options, if available for the year of bike buy OEM from a kawasaki dealer, or buy the non-OEM ones off ebay/wemoto etc... they work perfectly fine and are easier to remove for repair as you can use standard brake piston pullers to remove them.

They are for heat dissipation. FWIW Kawasaki do not recommend using OEM sintered pads on pistons without a heat dissipating insert as it could lead to excessive brake squeal - so they say.