I have one of the Hoppers kits on my XC GXL. Discovered after the callipers seized up that they have phenolic pistons, yes plastic pistons in them. I have never heard of such a thing, and I'm not impressed with the idea. What are People's experiences with them? I realise that they don't rust and transmit less heat to the brake fluid but what are the negative implications? I read somewhere they would never use them for racing. I won't be using them anymore either, now that I know they exist.