I agree on the pressure test. If the bike is new to you that could be left over for any reason. In that case I would clean the cooling system well, with Calgon autimatic dishwasher detergent. NOTHING else works quite like it and is the reason that Detroit Diesel specifies it for flushing their systems which have a pretty intricate oil cooler. start all over with fresh fluids of your choice. if you know it I a recent development without a fill problem etc, the water pump seal and shaft would be my first stop.