Daimler Trucks North America LLC (DTNA) is recalling certain model year 2008-2015 Thomas Built Buses Saf-T-Liner HDX school buses manufactured May 25, 2008, to December 18, 2014, and equipped with certain Cummins Westport ISL G engines. Excessive engine crankcase pressures may cause the 90-degree elbow of the vent tube assembly to detach from the crankcase ventilation breather, possibly allowing engine oil to come into contact with hot surfaces.