I was an apprentice at 15 yrs old (in 1961) in a garage that was a Girling and Lockheed brake specialists,I was taught to ONLY ever use red rubber grease on ANY brake or clutch master or slave cylinders.When I do my brakes,I remove the caliper,remove the pads and pump out the pistons one at a time until I see it is a clean area.I then get an old thick shoe lace and make a loop and wrap it around one piston at a time,then using a good blast of brake cleaner use the lace backwards and forwards to get the piston clean,then finish off with another good blast of brake cleaner.After it is dry use another lace smeared in red rubber grease wrapped around once more and spread a decent layer of red grease ALL around the piston before pushing back in.If you do that every-time even if they look as if they are clean,you shouldn’t have any piston seizure problems ever.I use hi-temp lithium grease on the back of the pads when re-assembling.The K2 hi-temperature grease is only for external sliders and the backs of brake pads and not for any seal or protective rubber that comes into contact with brake fluid.NEVER use copper grease on anything to do with brakes,it has metal in it to stop seizures that will destroy your highly polished pistons and caliper bores and doesn’t like being exposed to the weather.