This air brake relay valve fits Mack applications and is built as an aftermarket replacement for the original equipment. Positioned near the brake chambers rather than at the treadle valve, it applies or releases air faster, cutting lag time in the braking system. It is sold individually, one per package.Interchanges:Mack 745-104574LRV-3684FeaturesAir brake relay valveSold pack: 1 eachTech SpecsSell Pack: 1 Each