Abstract: The invention comprises a plug assembly for selectively closing and opening a flowway in a tubular conduit. The apparatus includes a seating body having a main passage and adapted to be mounted in the conduit with the main passage in register with the conduit flowway. The seating body defines a seat for receipt of a plug for closing the main passage and supporting a pressure differential across the seat. The apparatus further includes a release control mechanism operative upon the presence of a pressure differential across the seat in excess of a given magnitude to open a bypass passageway bypassing the seat and plug to permit a relatively slow decrease in the pressure differential across the seat.