Happened to me on my 1990 300SE except the failure was catastrophic - neck broke off and coolant spilled all over the place.

You can try to reinforce the neck with a piece of copper tubing if you can find one that fits snugly and use some high-temp epoxy like JB Weld.

There is a description of the problem and a repair kit at: http://www.**************.com/. It is in the section called 'Maintenance Alerts'.

Many threads on this problem here - a $200 radiator makes the problem go away for a long time. The wiki section has a replacement guide.