EE WAS OPERATING A MANBUS TRAVELING INSIDE AN UNOCCUPIED MANBUS COMING FROM SECT.#2 HIT THE FIRST BUS HEAD ON. THE PARK BRAKE OF THE 2ND BUSHAD BLED OFF, ALLOWING IT TO ROLL AWAY AT A DISTANCE OF APPROX. 5,440FT., HURTING HIS L.KNEEAND LOWER LEG. EE IS ON RESTRICTED DUTY AT THE PRESENT TIME.