On September 11, 1994, at 0830 central daylight time, a Wetherly 201C airplane, N9054W, operated by Larson Ag, Inc., sustained substantial damage when it departed the runway during takeoff near Stephen, Minnesota. The commercial pilot sustained serious injuries. Visual meteorological conditions prevailed for the aerial application flight conducted under 14 CFR Part 137. No flight plan was filed. During a telephone interview, the pilot reported the initial portion of the takeoff roll was uneventful. As the tail started to lift off, the airplane began to pull left. He said he tried the right brake and the airplane continued to deviate. He reduced the throttle to idle and applied "hard" right brake as the airplane departed the runway. The airplane traveled through a shallow ditch into a soft wheat field and nosed over. The pilot reported the left tire left a skid mark on the runway. Following the accident the left wheel rotated normally. The previous owner of the airplane, reported during a telephone interview, that on May 25, 1994, the airplane had experienced a similar occurrence. He said the airplane had nosed over during landing and had sustained minor damage. He reported that the nose over was attributed to failure of the left main landing gear trailing link. The airplane was inspected by a designated Federal Aviation Administration Aircraft Inspector. In his written statement, he reported that when he arrived at the accident scene, the airplane had been moved into a hangar. He said the left wheel turned freely and the left wheel brake operated normally. Further investigation revealed the parking brake control was improperly rigged. He reported that the improper adjustment of the cable prevented the cylinder from releasing completely, causing partial inadvertent application the parking brake.