Use alcohol or brake fluid DOT 3 or DOT 4 to clean the wheel cylinder and the piston.
CAUTION
In order to keep the piston cup from becoming twisted or slanted, slide the piston cup down special tool MB990619 slowly and carefully, without stopping.
Apply brake fluid DOT 3 or DOT 4 to the piston cups and special tool installer (MB990620).
Set the piston cup on special tool with the lip of the cup facing up. Fit the cup onto special tool, and then slide it down the outside of special tool into the piston groove.