The present invention relates to a tank piston with an improved seal having a cap, a body attached to the periphery of the cap, a circumferential channel adjacent to the periphery of the cap and an annular seal held within the channel by an annular sheet. In one aspect of the invention the annular sheet secures the seal within the channel and is attached to the piston by a ring proximate to the periphery of the cap and a ring proximate to the back side of the circumferential channel. In another aspect of the invention the annular sheet is made from a friction reducing material and enables the piston to more easily move within the tank and subsequently extends the lifespan of the seal. Additionally, the annular sheet is fitted to the seal and attached above the front sidewall of the channel.