HISTORY OF FLIGHTOn May 30, 2017, about 1600 Pacific daylight time, an experimental amateur-built Velocity SUV, N399DG, departed the runway after landing at Pierce County Airport - Thun Field, Puyallup, Washington. The pilot was not injured, and the airplane sustained substantial damage to the canard and both wings after striking an airport fence. The airplane was registered to, and operated by, the private pilot as a 14 Code of Federal Regulations Part 91 personal flight. The local flight departed Thun Field about 5 minutes before the accident. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot had purchased the airplane in Tennessee from its builder about one week before the accident, and spent the intervening period flying it back to his home base of Thun Field. He stated that during taxi after one of the return flight legs, the right brake became ineffective, and therefore he was unable to turn the airplane right. He inspected the brake system and was not able to find any anomalies, and on the next three flights, he could not duplicate the problem. On the day of the accident, he planned to fly the airplane in the traffic pattern. He performed a preflight inspection, and reported that during the engine ground-run he checked the brakes, and they held. Additionally, the taxi route from his hangar to the runway required multiple right turns. The takeoff, climbout, and landing approach were uneventful, and he touched down just beyond the runway numbers, at an airspeed of 82 knots. He applied pressure to the combination rudder/brake foot pedals to slow the airplane down, and once it had reached about 35 knots, the resistance in the right pedal suddenly dropped, and the pedal moved to almost full travel. The airplane immediately veered to the left, and the pilot released pressure on the left pedal. He began to "pump" the right pedal in an attempt to regain braking action, but the airplane did not slow down. As the airplane approached a runway light, the pilot applied left pedal pressure, and the airplane veered left, departed the runway, and struck the fence. TESTS AND RESEARCHBrake and Steering System The airplane was equipped with a castering nosewheel, with steering accomplished through differential brake pressure once rudder effectiveness had reduced at slower speeds. The brakes were activated by the pilot through the rudder pedals. The design did not incorporate conventional toe-brakes, but instead braking action was applied directly via the rudder pedals once they had been pushed about 2 ½ inches. The main landing gear struts were equipped with Matco W600 series brake and wheel assemblies, which incorporated a triple-piston brake caliper, and a steel brake disk which was attached to a threaded aluminum wheel hub by three hex-head bolts. Each wheel assembly was enclosed in a composite wheel pant, which covered the caliper and brake rotor. Post-accident examination revealed that all three hex bolts for the right brake disk were missing, and the disk had become detached from the wheel hub. The disk on the left side was still in place, but was loose, and the three bolts were finger-tight. The bolts and disks had holes to accommodate safety wire, but no safety wire was found on either assembly. Right Brake Disk Detached from Wheel Hub Maintenance Construction of the airplane was completed in June 2012, and at the time of the accident, it had accrued a total flight time of about 138 hours. Maintenance records indicated that it failed to pass its conditional inspection on May 19, 2017, due to the lack of an emergency locator transmitter (ELT). An entry by the builder dated May 26 detailed that he installed an ELT and completed a series of repairs and upgrades including the replacement of the brake master cylinders, adjustment of the main landing gear camber and toe-in, (due to uneven tire wear), along with modifications to the avionics system. The builder stated that the toe-in adjustment required removal of the brake assembly (including the three hex bolts) and installation of shims at the wheel axle mounting points. He could not recall if he had used safety wire to secure the hex bolts, or if he had ever used safety wire for their retention in the past. He further reported that the master cylinders were replaced because he encountered a loss of brake effectiveness in the right brake, which could be overcome by "pumping" the right pedal. The builder stated that all the work, except for the ELT installation, had actually been completed prior to the conditional inspection on May 19, but that he did not record the entry until one week later. On May 26, 2017, the same airframe and powerplant rated mechanic (with inspection authorization) who initially inspected the airplane, certified that it was airworthy. The mechanic reported that he had examined the brake system at the time of the initial inspection, but could not recall if safety wire had or had not been installed on the disk bolts. He did not re-examine the brakes during the follow-up inspection, as the ELT was the only item which required attention. During the 21-flight hour period leading up to the accident no other brake-related maintenance procedures were performed.