The first thing you need to do is pressure test the system to find the leak and repair it. It's entirely possible that you overheated it too much before and it now has a blow head gasket.

It may sound silly, but i did the same thing. After flushing out the system and replacing the radiator, I had no heat. I reversed the hoses on the heater core. They are the same size, but one lets hot water in, and the other ..you get the picture. After switching the hoses, i regained heat.

Reversed heater hoses will not cause coolant loss nor overheating. Heater hoses wouldn't need to be removed to change a radiator anyway..