The present application includes and assembly having a rigid hollowed body and an internal dart configured to traverse the length of a well bore and remove contaminants. The body having a central channel for accepting the dart. The dart includes a central passage and an exit port to selectively permit the passage of working fluid around and through the dart. The dart is configured to transition between an extended position and a seated position. The body includes one or more restricted diameter sections to create a dampener effect to minimize and distribute the impact forces upon the assembly. The assembly optionally includes one or more external wear pads, and an expandable seal coupled to the body configured to expand in diameter from the increase in pressure in the well bore. The expandable seal contacts the walls as the assembly rises in the well.