An adjustable sealing device for a storage container is disclosed that has a compressible sealing ball to provide a seal with the inner surface of containers having a variety of sizes and shapes. The device's valve ball for its vent is normally biased closed by a tensioner, and is selectively opened by a flexible lead. The lead passes through a container lid and compression washer(s) located between the sealing ball and lid. To insert the sealing ball into the container the lead is tensioned, to orient the longitudinal axis of the washer(s) perpendicular to the lid and sealing ball, so that pressure on the lid will be transmitted to the sealing ball through the compression washer(s). Releasing the lead's tension enables the lid to be placed on the container, and the lead and compression washers to be compactly draped over the sealing ball inside of the container.