It still has air in the system. You need to detach the coolant reservoir and hook it to the hood with a bungee cord or something. Refill the tank until it stops draining into the engine, once it's at the full mark. Turn off engine, re attach the tank and restart with heater in full blast to remove excessive air from the system and open the butterfly bleed... continuously check that it doesnt overheat, if it does, shut it off wait 20 minutes and restart once it returns to sorta operating temp. Same thing happened to my 02 Celica. No heat, coolant boils, gurgles and overheats... not enough coolant to cycle through and it had lots of air... whole thing took about an hour and a half since it kept overheating, but it's been fine since.
