On Thursday, February 8, 2001, at approximately 2:40 p.m., ten employees were transported underground on an Eimco Rail Runner Type II personnel carrier. One employee exited the vehicle at the No. 2 belt drive. While traveling down a 4.4 percent track grade inby the No. 2 belt drive, the service brake system failed due to a blown O-ring in the outby brake caliper causing a runaway condition. The operator attempted to stop the vehicle through the use of the dynamic electric brake and the parking brake. The mantrip traveled approximately 1190 feet before leaving the track at a turn and struck the coal rib. One employee was thrown from the vehicle and suffered a broken leg. Five employees suffered various strains, sprains, and chipped bones. Three suffered no major injuries.
The accident occurred because the brake pads on the outby service brake caliper had worn unevenly allowing the brake piston to overextend to a point where the sealing O-ring blew out. This caused a complete loss of service brake pressure to both front and rear brakes. Because the front and rear service brakes were operated from a single master brake cylinder, the loss of hydraulic fluid on the rear brakes rendered the entire service brake system inoperative.
Hydraulic service brakes should be designed so that the loss of pressure to a single brake line or caliper does not render the complete braking system inoperative. A single master cylinder that provides hydraulic pressure for the service brakes should be replaced by two master cylinders or a dual master cylinder to prevent this condition.