My father in law's Honda Civic overheats when the water reservoir is full or to the top. He is wondering what could be the problem if the radiator needs water when the reservoir is full. The temporary solution we found was to use a sponge or something to suck out the water from the reservoir to leave it leveled and pour some in the radiator. But it would overheat after a while due to the same problem.
We thought it could be a sensor but he changed it a while back. Also the thermostat is new the radiator as well and the hoses.