Yes it is a pump vent and they go bad. It will drip, drip, drip in the back. I tried JB Weld on mine and it did not work so I had Phil send me one from Fastlane. It was a cause of slow coolant leakage. Now I have no leaks in over 2 years. They are easy to relace. $37 I believe. U need drop leve a bit not entirely. Make sure u don't drop the sealing metal washers and clean mating surface good with some fine emory. It is super easy job, the kind we all like.

__________________
1982 300SD