This tractor protection valve fits Mack Universal applications and is a replacement, sold individually. It works by automatically shutting off trailer air supply if pressure drops suddenly, such as in a breakaway or major leak, keeping air in the towing tractor’s own system intact. It also fits certain Navistar applications.Interchanges:20QE3144Mack 20QE377745-279000745-280830745-280932745-288055750-2155400Navistar 776407C91LPV-3591FeaturesTractor protection valveFits Mack Universal applicationsSold as 1 EACHTech SpecsSell pack: 1 EACH