I had a 1977 240D and the blowby was so bad it soaked the air filter to the point of oil dripping on the ground. I believe later models had a chamber under the filter (intake manifold?) that let the blowby oil drain back into the oil pan.

I rebuilt the engine (pistons, rings, valve guides, valves, oil pump etc.), drove the car for another ten years (150,000 miles) and sold it on ebay when it turned into a rust-buggy...and it still had blowby...but it ran!

I think I would have to describe the cap (before rebuilt) as "blowing off" and after the rebuild as a "dancer"...

Hope this helps
The Tenor Man