A material distribution system includes: a chassis; a material tank carried by the chassis and configured to hold a material; and a header configured to receive and distribute material from the material tank entrained in an air flow. The header includes: a header body having a pair of sidewalls, an inlet configured to receive entrained material from the material tank and a plurality of outlets, the sidewalls defining a width therebetween that increases from the inlet toward the plurality of outlets; a valve associated with an outlet that is configured to switch between a distributing position and a purging position, the valve blocking its outlet in the purging position; and a plenum having a volume of air that is selectively in fluid communication with the plurality of outlets to provide a purging air flow to the outlet with the associated valve when the valve is in the purging position.