A relief valve (60) for a liquid container, the valve comprising a substantially conical body (50) extending into the container, and tapering inwardly away from an external wall of the container, at least one air inlet passage (62) through the substantially conical body; and a flexible diaphragm (53) extending over the conical body, being attached at its outer periphery around the substantially conical body and having a central opening surrounding the substantially conical body above the air inlet passage so as to seal the air inlet passage until the pressure within the container falls below a predetermined level.