On June 1, 2011, about 1640 Pacific daylight time, a Cessna 340A, N340BG, was substantially damaged when its nose gear collapsed and deformed primary airframe structure in the wheel well. The event occurred when the pilot lost directional control as he taxied off runway 24 following an uneventful landing at the Fullerton Municipal Airport (FUL), Fullerton, California. Neither the airline transport certificated pilot nor passenger was injured. The airplane was registered to and operated by Humanitarian Airlift International, Coto de Caza, California. The personal flight was performed under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed. The airplane departed Riverside, California, about 1545. According to the pilot, following completion of maintenance he took off to reposition the airplane. No evidence of mechanical anomalies were noted, and his landing at FUL was normal. Near the completion of the landing rollout, with normal and effective breaking, the pilot initiated a turn off the runway. During this event, the left brake system failed, and the airplane veered into soft dirt adjacent to the runway. The nose wheel dug into the dirt and collapsed. During the airplane's subsequent examination by an airframe and powerplant mechanic, a broken weld was found in the left engine's oil return line. The weld was located adjacent to the line's outlet nipple at the rear engine case. The line was routed to the case from the vacuum pump air-oil separator. Leaking oil from this broken weld had coated the rear engine case and left main landing gear brake assembly. According to the mechanic, engine oil on the left gear and wheel/ brake assembly caused braking action on the left brake to be reduced to zero. The weld break appeared related to fatigue.