Abstract: On September 19, 2008, Employee #1 was working alone in a maintenance and repair shop. He heard the sound of air under pressure leaking from beneath a truck trailer. He found that air was escaping from an air brake chamber and was probably the result of a loose retaining ring that held the two metal parts of the chamber together. Working from the side of the air brake chamber, he tightened the nut on the retaining ring but the air leak was not eliminated. Therefore, he continued tightening the nut on the bolt until the bolt broke. Because the chamber was under air pressure, the retaining ring released and one half of the metal air chamber blew apart. The piece of metal chamber struck the employee in the jaw. Employee #1 was hospitalized. The chamber was under pressure because it needed to be bled before opening the chamber.