Abstract: A plunger for well casings and other tubulars comprising: An elongate body having a flow passage with a lower inlet port and an upper outlet port; an internal valve positioned between the ports; at least one first flexible sealing member positioned around the body below the outlet port; an actuator which urges the first sealing member against the interior wall of the casing when the valve is closed; at least one second flexible sealing member positioned around the body and providing an outwardly expandable sealing chamber; and at least one flow port positioned below the valve and providing fluid communication between the flow passage and the sealed chamber. The second flexible sealing member(s) being outwardly expandable by increasing pressure in the sealed chamber.