On June 22, 2018, about 1730 Alaska daylight time, a tailwheel equipped Cessna 180 airplane, N3125D, sustained substantial damage during an off-airport landing at Upper St George Creek, about 50 miles south of Fairbanks, Alaska. The airplane was registered to and operated by the pilot as a 14 Code of Federal Regulations (CFR) Part 91 visual flight rules personal flight when the accident occurred. The private pilot and passenger were uninjured. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight departed Fairbanks International Airport (PAFA), Fairbanks, Alaska about 1700. According to the pilot, he was landing with a right crosswind and after touchdown, as the airplane began to decelerate, he applied left brake in an effort to maintain directional control. He stated that the left rudder/brake pedal was "soft" and traveled to its full forward limit and directional control was lost. The airplane weathervaned into the wind and skidded sideways before the left main wheel went off the cleared landing area and into brush. The left wing and left horizontal stabilizer subsequently impacted terrain, which resulted in substantial damage. Upon exiting the airplane, the pilot found the brake line had separated at the compression fitting near the brake caliper where the brake line transitioned from a rigid line to a flexible line. The brake line tube and fitting hardware was removed from the airplane and sent to the NTSB Materials Laboratory in Washington DC for further evaluation. The interior surface of the failed end had circumferential marks up to 0.136 inch from the end consistent with the formation of a flared end with a flaring tool. The sleeve was trapped within the nut from the failed end, and deposits of gray metal appearing consistent with deformed brake tube material was observed deposited on internal threads of the nut up to 3 threads from the end of the sleeve. According to Federal Aviation Administration Advisory Circular AC 43-13-1B, Acceptable Methods, Techniques, and Practices – Aircraft Inspection and Repair, a double flare is used on soft aluminum tubing having a 3/8-inch outside diameter or less. No evidence of folded walls consistent with a double flare was noted at either end of the submitted brake line tube. No records could be located that showed the manufacture or assembly date of the fractured brake line components.