Only fill the radiator so it just covers the fins. If you overfill the water gets pushed out and once flowing just seems to keep on emptying. The radiator cap do's not have a rubber seal but seals on the brass surface of the cap. My old QL and OXC threw its water out if I overfilled but was never a problem if I kept the water low. A thermostat in the system seemed to make it worse, I never had one fitted for this reason,