The fittings that connect lines in an air brake system are vital to the control in stopping the vehicle and are referred to as D.O.T. fittings. Because of the importance of these connections these D.O.T. Fittings are compliant to Department of Transportation standards. Push In Union Tee Fittings are used for a variety of applications including connecting a new tube line to an existing tube line at a 90 degree angle.