D.O.T. (Department of Transportation) approved Push to Connect fittings are the most commonly used connection in air brake applications. The compact style of a Push to Connect fitting is well suited for installing in confined areas. However D.O.T. regulations require all nylon tubing air brake connections have fittings with an inner tube support. The tubing slides into the fitting and the metal inner tube support slides into the tube. This provides a more solid connection for over the road air brake systems.