I have succesfully used a plastic stiff pipe to measure the fluid level, it is important to measure it hot.

For cooler flush I like the lubegard kleen it product in an aerosol can, In the newer MB they are actually plate n fin coolers in the radiator tank rather than the old simple copper tube like we saw in the old W123 etc cars

In most cars that I have used that flush quite an amount of debris also was washed out of the cooler.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)