This air brake relay valve fits Mack Universal applications and is built as a replacement for the original valve. Mounted in the brake system, it applies or releases air to the brake chambers on command, cutting lag by working closer to the chambers than the treadle valve. It is sold as a single unit.Interchanges:Mack 745-280375LRV-3617FeaturesRelay valve for air brake systemsFits All Mack Universal applicationsSold pack of 1 EACHTech SpecsFitment: All Mack UniversalSell pack: 1 EACH