This relay valve is built for heavy-duty air brake systems on Mack and Navistar trucks, sold individually as a one-each replacement. Mounted near the brake chambers rather than at the treadle valve, it applies and releases air on demand, cutting the lag that comes with routing air all the way back to the treadle valve. It’s a straightforward swap for trucks specifying the original Mack or Navistar valve.Interchanges:Mack 745-103010Mack 745-103834Navistar 1677540C91LRV-3507FeaturesRelay valveSold individually, 1 EACH sell packDesigned for heavy-duty air brake systems