Outfit your semi to be Ready for the Road Ahead with enhanced air dryer performance, with the Bendix 8644 Series 4" x 4" Air Dryer Purge Valve. Thanks to its specialized design, this replacement valve is the ideal choice to release moisture that has been captured in the air dryer system. Crafted with durable materials and a resilient design to operate in temperatures as low as -40 °F. FleetPride is your First Click in Heavy Duty .