A pressure-relief valve assembly is improved in that sealing surfaces on both the valve seat and valve head are confrontationally disposed annular planar surfaces oriented substantially perpendicular to the valve axis. A resilient valve seal is formed of a material which deforms to a predetermined extent into sealing engagement against both sealing surfaces when compressed by the spring biasing force. When a vacuum exists the combination of the spring force and a holding force created by a pressure differential acts to increase the extent of compression between the resilient valve seal and both sealing surfaces sufficient to create a vacuum-tight seal. The magnitude of the spring biasing force in discrete increments may be adjusted by varying the degree of threaded engagement between members forming the valve housing.