The airplane was returning from a retardant drop on a forest fire when the crew noted that the airplane’s main hydraulic system had failed. They configured the airplane using the emergency systems and declared an emergency. After landing on the runway, the pilot elected to exit the runway using the high-speed taxiway that went to the tanker base ramp. When he tried to stop the airplane using the emergency system brakes, there was no braking response. The airplane crossed the tanker ramp, exited the prepared surface, and went through the airport perimeter fence, down an embankment, and onto a road. According to the airplane flight manual, the procedure for a hydraulic system failure is to land and stop on the runway using the emergency brake and accumulator pressure. A postaccident examination of the airplane’s hydraulic system showed a rupture in the line that interconnects the main hydraulic system and retardant tank system. No other systems anomalies were found. The crew later reported that when the co-pilot lowered the nose gear using the emergency gear extension system, she did not return the emergency nose gear extension system selector to the “neutral” position, and it remained in the “bypass” position. This shut off the emergency system hydraulic system pressure to the elevator flight control and emergency brakes. Probable cause: “The pilot’s failure to follow published emergency procedures by taxiing to the parking ramp with a known hydraulic system failure. Contributing to the accident was the co-pilot’s improper selection of the bypass position on the emergency nose gear extension system, which shut off emergency hydraulic system pressure to the brakes, and a ruptured hydraulic line, which resulted in a total loss of the main hydraulic system pressure.”