Slipper seals use a combination of a wear band on top that provides the sliding or dynamic surface. Underneath the wear band, a compressed O-ring is positioned. In very high pressure applications, the O-ring may have back up rings on either side. This seal design provides a cost effective and long life dynamic seal solution. It is not as leak tight as cup style seals but is able to endure heavy side loading. The slipper seal design is also very space efficient allowing for a very short piston design or enabling the piston designer to incorporate large bearing surfaces on the piston. (http://vphydraulic.blog...)