Certainly seems to be a safety switch issue. Does the engine die as soon as the brake is released? there are 3 switches in the safety interlock system. The PTO Switch, The Brake Switch, and The Seat Switch. I own a 455 and have had problems with the seat switch. Remove the connector and reconnect, rapidly actuate and de-actuate the switch. If all else fails ohm out the switch to see if it has failed.