THE PLT RPRTD THAT DURING THE TAKEOFF ROLL, THE ACFT ACCELERATED TO ABOUT 65 KTS. HOWEVER, AS HE ROTATED THE PLANE FOR TAKEOFF, IT BEGAN TO SLOW & VEER TO THE LEFT. THE PLT REDUCED POWER TO ABORT & REGAINED DIRECTIONAL CONTROL, BUT THE BRAKES WERE INEFFECTIVE IN STOPPING. SUBSEQUENTLY, THE ACFT CONTD OFF THE END OF THE RWY, THEN HIT A FENCE & A PARKED VEHICLE. THE PLT BELIEVED THAT WHEN HE RELEASED THE PARKING BRAKE, IT DID NOT FULLY DISENGAGE, THEN HEAT BUILT UP AS THE BRAKES WERE DRAGGING. AN EXAM OF THE BRAKES CONFIRMED THAT HEAT DAMAGE HAD OCCURRED.