While I personally am not a fan of phenolic brake pistons, auto manufacturers continue to use them in heavy duty applications such as Police Package vehicles... They have always had a higher occurance of sticking in calipers than steel pistons...And everytime I've taken one apart it seemed to have deposits on the sides of the piston... FWIW Glad the calipers looked good, sorry I wasted your time But at least it won't cause you to lose any sleep...