It is a banjo bolt. It has a metal washer with a rubber sealing lip on the inner circumference. I fought this leak for quite some time a couple years ago. The ONLY way to seal it up is with the OE sealing gasket/washers, P/N MB033054. I'd also recommend replacing the banjo bolts themselves, P/N MB033025. I ended up replacing the cooler lines themselves after the leaks persisted between the loose cover on the hose and the hose itself.

They're not cheap, but $100 or so is worth alot of time IMHO. I wish like hell somebody would've given me the info I'm giving you, back then. I wasted alot of time on this. You can try to fix it if you're broke... <img src="/forums/images/graemlins/cyclops.gif" alt="" />