In my house there is a towel heater radiator in the bathroom that is only half full the bottom half is hot and the top half has no water in it therefore cold. it is not bleeding no air or water is coming out the top. I have tried a variety of methods like filling the radiator up with water from the top stabbing down the bottom to check for blockages but nothing. when i filled it up with water and opened the valves the water went down to the same level it was before. the heating is working off the old gravitational system. please give me some adivice so I can solve this problem