I think people are confusing what happens with a conventional US car (unpressurized) coolant overflow tank vs. the Mercedes pressurized tank. In the US system, the car burps fluid out under pressure into the overflow tank, then pulls it back when cooling off. The Mercedes system is simply a radiator with part of it (the plastic tank) being "see through". In the US system, there is not supposed to be much air in the pressurized area. In the Mercedes system, there is significant pressurized air inside the system.
Since people that have this engine are telling you that this is normal, and you also indicate the radiator cap is now new, maybe what you need is a new water pump and not a new head?
I know the dealer will not be as happy........
So, we should get more people in here that actually have a 603 or 602 engine to tell you their experience. Maybe we both have cracked heads (in the engine).