I've used a product called CLR. It's a household cleaner that's commonly used to remove residue from hard water off windows or shower enclosures. Just soaked the pistons overnight and clean them with a parts cleaning brush. Rinsed in warm water and let them dry. Got the suggestion from a car restoration guy. I had to do some pistons twice, but hey, it took them 50 years to get crudded up that bad. The stuff softened up the carbon great.
