I recently replaced my radiator & I'm still losing coolant. The heat works & I don't smell any coolant leaking while it's running. I've had the cooling system pressure tested repeatedly & there is no leaks & the overflow reservoir stays full. Yet every 1/2 tank or so I need to add about 1/3 cup to the radiator? I can't see any leaks when I check the car after running it for a while.