On July 19, 2016, about 1120 central daylight time, a Vought F4U-5 Corsair airplane, N179PT, departed the runway surface after landing at the East Troy Municipal Airport (57C), East Troy, Wisconsin. The pilot was not injured and the airplane was substantially damaged. The airplane was registered to Fighters & Legends LLC and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight. The local flight departed 57C about 1115.The pilot departed from 57C on a maintenance flight, in order to test the airplane brakes. He reported that the brake tested normal during the taxi. He applied the brakes several times in flight, and received positive pressure of the brake pedals. On the full stop landing to runway 8, the pilot applied the brakes and received normal braking action. As the airplane slowed, it slowly drifted to the right. The pilot applied a small amount of left brake to correct the drift and the pedal went to the floor; pumping the pedal did not correct the problem. In order to avoid a ditch, the pilot applied the right brake; however, the airplane's wing collided with the airfield's windsock. Substantial damage was sustained to the airplane's right wing. Prior to the accident, the airplane's brakes were replaced with a custom brake system using Grove master cylinders. Testing of the brake system after installation resulted in a failure of the right master cylinder. The cylinder was disassembled and the O-ring was found cut. The mechanic could not find any reason for the cut O-ring, so the O-rings on both master cylinders were replaced with Viton O-rings and care was given to carefully place them into the cylinders. The next brake test resulted in a brake fluid boil, so the brake builder informed the mechanic to change the hydraulic fluid from MIL-PRF 5606 to MIL-PRF-83282. A subsequent ground brake test produced "no issues or hesitation with the brakes at all" to include "a full pressure pedal push to simulate a full locked brake to pressure test [the] system prior to taxi test." After the accident, the mechanic contacted the master cylinder manufacturer (not the brake builder) for guidance. The company replaced the master cylinders with an upgraded model. The new cylinders were installed on the accident airplane and the mechanic, with guidance from the custom brake manufacturer, conducted more testing. On February 6, 2017, a second Corsair flew with the newer brake system without issue.