On June 9, 2022, about 1145 mountain daylights time, a Cub Crafters, CC19-215, N61KT, was substantially damaged when it was involved in an accident near McCall, Idaho. The pilot and flight instructor were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The pilot of a tailwheel airplane reported that, while attending a backcountry flying course, he had completed several off-airport landings with a flight instructor before returning to McCall Municipal Airport (KMYL), McCall, Idaho. During the landing roll to runway 34, while applying brakes, the left brake “flopped” under his foot with no pressure and the airplane veered to the right. The pilot was unable to maintain control of the airplane and it traveled off the side of the runway and down an embankment, where the left wing and tail contacted the down-sloping terrain. The pilot was then able to taxi the airplane back to the ramp. Postaccident examination by a Federal Aviation Administration inspector revealed that the left brake line near the caliper was separated and the left wing and elevator were substantially damaged. The brake caliper and brake line components from the left main landing gear were sent to National Transportation Safety Board Materials Laboratory, Washington, DC, for examination. The brake line 45° fitting at the lower side of the caliper was fractured. Examination of the fracture surface revealed characteristics consistent with a ductile overstress fracture. Additionally, a sliding contact mark was observed on the lower surface of the caliper aft of the fitting. The surface within the contact mark was smeared and exhibited a deformation pattern consistent with contacting an object moving forward relative to the brake caliper. An additional contact mark was also observed on the aft side of the fitting adjacent to the sliding contact mark on the caliper.