2001 Hyundai Tiburon Vehicle Overheating

I have a 01 hyundai tiburon, manuel, my vehicle keeps on overheating after i had fixed my cylinder heads, and water pump about six months ago. i kept on putting water in the radiator and took out the thermostat. a couple days ago i decided to put the thermostat back in and use 50/50 mixed. however, my vehicle still overheats. The radiator does have rust in it and the coolant drains constantly. Any ideas as to why my coolant drains quickly.