seems like you will need to fill in and get a pressure tester that you can pressurize the system to rad cap value and look for leak. or you can make a tee that goes in the hose that has a shreader that you can fill with air with a tire chuck to the radiator cap value and search for that source of water leak.
/ M6800 got hot, small pop noise, then lost some coolant and steamed.
I suspect it is built in to the radiator cap on that model. Is there an over flow tank connected to the neck of the radiator where the cap screws on? If yes then cap is the relief.
/ M6800 got hot, small pop noise, then lost some coolant and steamed.