The way is to start with the longest brake line 'measured along the line' from the master cylinder. This may vary on different cars.. it can be routed differently from one model to another.. from one year to another... so best to look at how it gets to all the brakes...
When you do it this way after the first one there is clean fluid at the start of their line...
I do not know enough about MB to say which way it is... this rule applies to all cars defined this way...