A pressure relief valve for use with flexible containers is disclosed. The valve comprises a valve chamber, a valve chamber seat with an inlet gas port or ports to the interior of the container and a resilient diaphragm floating within the chamber. The valve seat is shaped with a generally flat diaphragm-engaging surface in a central area of the diaphragm where the inlet gas port(s) are located, and at least one surface that does not generally engage the outer portion of the diaphragm, such that a diaphragm floating in the valve chamber engages a central portion of the valve seat surface stopping the passage of gases in or out of the container before a certain pressure is reached, but permitting the passage of gases from within the flexible container as pressure builds, while preventing the flow of atmospheric gases into the container.