We have a tool that vacuums down the system and sucks in the water. This prevents air bubbles. If you don't have one, you should pull the hose from the radiator side and fill slowly until a large quantity of water comes from the radiator. Turn on the heat to full and start the car. Run at 2000 till you feel heat.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS