A combination pressure-vacuum relief vent-valve for use in preventing excess pressure or vacuum within a railway tank car. The vent-valve includes a valve chamber having a first end adapted to communicate with the interior of the tank and a second end adapted to communicate with the atmosphere. Valve plug means is positioned within the valve chamber having a first position preventing fluid communication therethrough and a second position permitting fluid communication therethrough. The plug means is spring biased towards its first position in a manner which permits the plug means to assume its second position, and thereby relieve vacuum in the tank, only when the pressure differential between the first and second ends of the chamber exceeds a predetermined value. The plug means further includes a frangible element associated therewith to relieve excess pressure build up in the tank.