This quick release check valve is a replacement for Mack and Navistar air brake systems. Mounted in the air line near the brake chamber, it vents air locally on release, cutting the lag that comes from long return runs back to the treadle valve. It is sold as a single unit, one per pack.Interchanges:Mack 745-289182Navistar 1676744C1FeaturesQuick release check valve for air brake systemsSold pack: 1 EACHTech SpecsSell pack: 1 EACH