At about 11:15 EST a Cessna 310R on a training flight was damaged when it departed the taxiway at the Sullivan County International Airport. No one was injured. The flight had originated in Teterboro, N.J., and was being taxied back to the runway for the return flight with the second pilot, who was in the left seat, at the controls. As the second pilot turned to the runway hold short line, he discovered there were no brakes. The instructor then took the controls and depressed his brakes with no result. The airplane continued to roll, veering to the right into a grass ditch about 20 feet from the taxiway. The airplane came to rest straddling the ditch while being supported by the nose and tail. There was a maintenance discrepancy written prior to the accident flight regarding the airplanes brakes. During the corrective action to fix the discrepancy, a faulty Lock-O-Seal was found and replaced.