[Early_S_Man]

Specifically, Berryman's Chem-Dip! It wouldn't hurt to brush and turn over the piston a few time during the soak to give 'fresh' cleaner access to the carbon and lead deposits ... aeration/bubbles from a steel line submerged under the piston and supplied with air from an aquarium pump could keep the cleaner moving and improve the rate of loosening. Be sure to wear long-cuffed, heavy-duty rubber gloves and apron while handling the piston around the Chem-Dip ... or you will be running for a faucet in pain when it gets on your skin!!!

Rinse off with water and bristle brush should remove the carbon very easily. Good luck!