Originally Posted by lasermike

This.

It should sit high enough to cover the core and will look low to someone used to modern cooling systems. If you add enough to bring it to the neck and it drops back down to just above the core and stays there, it's working as it should. The exception is if you have a functioning coolant recovery tank and a cap with a good seal. My radiator level is right at the cap with the recovery tank sitting between cold and hot fill.

You probably had air in the system that kept the thermostat from opening in time or at all. That will lead to high pressure in the cooling system, 15 psi for a standard cap. I once blew the upper radiator hose due to a stuck thermostat. That might be enough to push water into a cylinder or past a weak intake gasket and cause white smoke. In my case, it was pushing past the intake so a compression test wouldn't have shown a leaking head gasket.

I've recently become a fan of vacuum cooling system fillers. They use shop air to remove the air in the cooling system then use that vacuum to pull in coolant. It even caused a freeze plug that was almost rusted through to show itself. With pressure, there was enough gunk in the system to plug the leak but when I pulled a vacuum on it, the plug began leaking right away not letting the vacuum build up. The system had held pressure with the pressure tester just before pulling the vacuum.

Michael