I have far more experience here than I wish .

I bought a 1979 300CD that was allowed to sit for well over a year with only tap water in the cooling system plus my 240D had 'solder bloom' .

Anyways : IMO the best way to clean the cooling system is buy driving the car with the citric acid cleaner in it, this may take a while like several days of driving to work and back before all the crud comes loose .

The heater matrix (core) is designed to retain coolant even with the hoses removed .

I disconnected the three heater hoses and used a garden hose to make the initial flush, I discovered that each time I reversed the flow more and different crud came out .

After it stopped spewing reddish water I closed the system and made a mix of citric acid powder and distilled water and ran that, it took some time but eventually the mix turned black and the inside of the radiator turned copper color again .

Then I flushed the system with baking soda and distilled water to neutralize the acid before filling the system with a 50/50 mix .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better