Wipers are used to clean and lubricate the exposed portions of piston shafts. They prevent dirt from entering the system and help protect the piston shaft against scoring.
Wipers may be either metallic or felt. They are sometimes used together, a felt wiper installed behind a metallic wiper.
Read more http://www.flight-mechanic.com/nonmetallic-aircraft-materials-part-six-seals-part-two/