A pipeline pig that can traverse dual- or multiple-diameter pipes or pipelines has collapsible and expandable sealing elements that support the pig, provide a sliding (dynamic) seal with the pipe wall, and maintain the necessary pressure for driving or cleaning as the elements collapse and expand. The sealing elements each have an internal cavity sized to house a first volume of a fluid and a reservoir sized to house a second volume of the fluid. The cavity and reservoir are arranged so that a portion of the volumes of the fluid can move between each. An accumulator maintains a pressure of the first and second volumes of the fluid so that regardless of the sealing element's current size, or whether the element is transitioning between sizes, the element can maintain a sliding seal engagement with the pipe wall and perform the primary function of driving or cleaning (or both).