I noticed on my 95 E420 with same problem. But i had my male fitting fall into radiator. and I had to take radiator out to get the broken part out. But this forum is helpful. The experience and suggestions are greatly appreciated.
2005 Triumph Bonneville T100 (Black/white) [Sold]
2007 GL450 (Dersert Silver)
2009 Toyota Corolla (Metallic Silver)
2002 Tundra V8 - 2WD (Desert Sand)
1995 MB E420 (Silver Smoke) [crashed, now resting in peace at Silverstar Recycling Center]