That is a very common problem with the MB radiators with plastic tops and bottoms. The top part is the part that gives the problem. It's the fitting for the top radiator hose, and all the ones I've seen, the owners were using Valvolene's Glysantin G05. I have replaced three radiators with the same problem and with the same coolant in the last nine months,and will be replacing another one next week. I have owned Mercedes for the last 34 years. but I won't use the antifreeze they recommend. I'll stick with Prestone.
Bill Lewallen Lexington,Ky. Home of Valvolene Oil