Abstract: A tank piston is shown with an improved seal and wiper. The piston is used in conjunction with tanks used for transporting semi-solid and viscous materials such as grease, oil, ink, and the like. The improved seal consists of an annular rubber member with a hollow chamber filled with an open cell foam material such as polyurethane, or a gel such as silica gel. The material in the chamber is compressible and expandable to provide a seal about the piston. Optionally, the piston is provided with an annular wiper structure that extends about the outer forward periphery of the piston, and is forwardly angled at about 45° relative to an axis of the piston. The wiper has a beveled end that makes contact with the interior surface of the tank to provide a cleaning action. A complementary second wiper may be added near the outer rearward periphery of the piston. A preferred embodiment is also shown that uses a tiered support assembly.