Moisture can enter the gearbox/parkbrake housing causing the water to freeze thus locking the parking brake discs in place. The best solution would be to remove the drive motor and the brake housing cover and clean the housing. While in there you should see one or two holes that have magnets around them. They are probably "fuzzed" over not allowing oil to flow in and out of the brake housing. This is not a difficult fix especially since the 865 was a single speed.