A method of making a plurality of flexible containers (12/112/212) comprising the steps of manufacturing a plurality of bag structures (13/113/213), separately manufacturing a plurality of valves (10/110/210), aligning the valves (10/110/210) with openings (24/124/224) in the bag structures (13/113/213), and securing the aligned valves (10/110/210) to the bag structures (13/113/213). Each valve (10/110/210) comprises a vent layer (30/130/230) which is pervious with respect to expected gasses and a sealed passageway is formed between the vent layer (30/130/230) and the container (12/112/212) when the valve (10/110/210) is secured to the bag structure (13/113/213).