An employee was connecting a train to standing rail cars on a track. He connected air hoses for the brakes to the car ahead of it so the brakes could be released. He then opened the angle cock (2-stage valve to give air or take air away from the system) and heard an air leak, so he closed the angle cock and tightened the gladhands. The angle cock was opened again and the gladhand coupling disconnected. Up to 90 psi of air that was trapped in the line caused one of the gladhand connectors to strike his right shin, causing a laceration and a fractured tibia. The employee was hospitalized.