On September 25, 1995, about 0830 mountain daylight time, N10DF, a Cessna 340, operated by American Check Transport, Inc., collided with a ditch during taxi and was substantially damaged in Riverton, Wyoming. The commercial pilot and his passenger were not injured. Visual meteorological conditions prevailed and a company flight plan had been filed. The cargo flight was destined for Worland, Wyoming, and was conducted under 14 CFR 135. The pilot reported that the left brake "failed" as the airplane was taxiing for takeoff from runway 28 at the Riverton Regional Airport. The pilot stated: About 30 - 50 yards from end of taxiway I discovered the left brake was [inoperative]. I attempted to make the 90 [degree] left turn to the runway but was unable and went off the taxiway about 10 - 12 feet where there was a ditch. The nose wheel [collapsed] when it hit the ditch [and] the nose of the aircraft was damaged.... The pilot also stated that he attempted to turn the airplane with power onto the runway when the failure occurred. An FAA aviation safety inspector from Casper, Wyoming, examined the airplane. He reported that the underside of the fuselage structure was substantially damaged. He also reported that hydraulic brake fluid had "leaked out" of the left brake system prior to impact with the ditch, and there was not enough fluid left to operate the brake. The inspector was unable to determine the source or cause of the leak.