Assuming you're talking about a W123? It is just a hollow ferrule nut (similar to brake line nuts) the tube passes thru and there is a brass compression nipple with hose barbs that pushes into the tube. The other end of the nipple is just a bowl shaped compression fit, like what is used on the old propane bottles or oxyacetylene bottle connections. Every time I've gone to loosen one it has swelled in there pretty tight from heat cycling, so be careful getting it off, I would suspect the tube is just getting loose on the barbs.

__________________
Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale