To clean the cylinder, also known as the piston block, I like to use some solvent on a brush and scrub in there real well. Then I take a rag with some solvent on it (see photo) and feed it through there, and work it back and forth shoeshine-style. Keep it up, until it's clean.
Make sure you wipe off all the solvent after cleaning, and give each part a light coat of oil before you put things back together.