A pressure relief valve for a reservoir has a main body with a central chamber defining an outer radial seat, a positive-pressure port with an inner radial seat, and a negative-pressure port comprising an internal passage spanning the outer seat. The sealing disk has an anchor body retained in the central chamber and a flexible diaphragm biased against the outer seat. An axial passage in the anchor body couples the central chamber to ambient atmospheric pressure. The sealing ball is biased against the inner seat. When a pressure inside the reservoir exceeds ambient by a positive-pressure threshold, then the sealing ball lifts off the inner radial seat to exhaust a gas through the positive-pressure port. When the pressure inside the reservoir is below ambient by a negative-pressure threshold, then the diaphragm is lifted off the outer seat to intake atmospheric gas into the reservoir.