On August 21, 1998, at 1545 central daylight time (cdt), a Cessna 150, N1239Y, piloted by a private pilot, was substantially damaged when it nosed over when it collided with a ditch shortly after departing the left edge of Runway 21 (2,905' X 40', dry asphalt) at the Lamar Municipal Airport, Lamar, Missouri. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 flight was not operating on a flight plan. The pilot reported no injuries. The flight departed Pittsburg, Kansas, 1515 cdt. According to the pilot, the wind sock at the landing was "...limp." She said she landed the airplane in a 3-point attitude and was on an uneventful landing roll when the airplane suddenly veered to the left. She said right rudder correction did not stop the turn. The ditch N1239Y collided with was about 30-feet from the runway edge and about 4-feet deep. According to the Federal Aviation Administration Principal Maintenance Inspector (PMI) who examined N1239Y, "...the [left main landing gear's brake] torque plate holes and torque pins were dry... ." The PMI said, "...a person who had assisted in the recovery of the airplane stated that after the airplane was [being] towed... he noted that the left main wheel would not rotate." The PMI said the dry torque plate and pins "...[have] been observed to cause locking of brake assemblies in this inspector's experience." Statements from the mechanic and PMI are appended to this report. The PMI said lubrication of the torque plates and pins normally should take place during the annual inspection and whenever the brakes appear to drag during service. The pilot/owner said she had not experienced any dragging brakes during the time she flew the airplane.