On July 14, 2015, at 1030 mountain daylight time, a Dafoe TCOW Smith Cub, N810GL, ground looped after landing, collapsing the right landing gear and substantially damaging the right wing. The private pilot was not injured. The airplane was registered to the pilot, and was operated as a personal flight under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the flight, and no flight plan had been filed. The pilot stated to the NTSB investigator that after landing he applied brakes to slow the airplane. He did not feel any right brake resistance, and the airplane swerved left due to left brake application. He attempted to correct with rudder but the airplane ground looped to the left, collapsing the right landing gear, and damaging the right wing. An FAA inspector examined the airplane and the brake system. He did not note any unusual wear on the pins or brake pads. He did note that the owner had installed an additional brake fluid reservoir to the right brake master cylinder. There was no indication of brake fluid leak. The pilot/owner did state to the inspector that he had been having on-going issues with the right brake. The NTSB investigator authorized the pilot to disassemble and examine the right brake system. The pilot reported back that he found a gouge in the master cylinder piston o-ring that would allow brake fluid to bypass the seal.