The commercial pilot stated on the Pilot Operator Report (NTSB 6120.1) that he had attempted and aborted a takeoff from another runway. On the second takeoff attempt the pilot stated that he decided to abort due to insufficient airspeed when the left brake faded out and the airplane gradually turned to the right and the wheels sank into soft mud and into a ditch. Substantial damage was sustained to the left wing spar which was bent approximately 20 degrees up. The main landing gear was also damaged. There was no post-crash fire and the pilot reported no injuries. The pilot reported his takeoff weight at just 55 pounds less that the maximum allowable gross weight of 6,000 pounds and said one of the reasons for the accident was that he should have loaded a lighter load.