A pressure or vacuum relieving device intended as a large capacity inbreathing or outbreathing apparatus for equipment and/or piping, operating under low to moderate gas pressure or vacuum, e.g. 1 to 15 psig pressure or 0-5 psi vacuum. The principal feature is that a required set pressure is provided hydrostatically by means of a liquid leg acting an a bellows expansion joint. The device is suitable for use in Pulp Mill Digester Blow Steam Systems and similar applications.