Abstract: A mold half carrier has first and second air flow compensating surface areas. The second air flow compensating surface area encloses the first air flow compensating surface area. The first air flow compensating surface area provides pressure compensation for formation of smaller containers. Both air flow compensating surface areas are utilized for formation of larger containers. A shut off valve controls air flow through a passage in the mold carrier extending between the first and second air flow compensating surface areas. The second air flow compensating surface area has an exhaust passage for bleeding unwanted pressurized air therefrom when the shut off valve is closed. The compensating surface areas are separated from each other by a common continuous groove and an O-ring mounted in the groove. The groove has an arcuate main portion for receiving the O-ring and an auxiliary portion into which a portion of the O-ring may collapse.