A sealing cap is provided for use in conjunction with safety storage containers and the like, which provides automatic venting and relief of interior vacuum, including low interior pressure resulting from contraction of the contents of the storage container, and from removal of liquid contents from the container. The cap provides a secure closure for a first opening in the container through which contents may be added or removed, while simultaneously providing the aforedescribed venting function. Incorporation of the present invention with a storage container having a second fluid dispensing opening avoids the necessity of providing an alternate or specialized air vent to admit air and prevent a vacuum build-up during dispensing of the contents. In one embodiment, the device is particularly immune to the effect of corrosive fluids stored within the container.