It is not a fuel line between the filter and the canister. I believe it is breather that allows the diaphragm that regulates pressure in the filter to work and breath or oscillate properly. IF the line between the filter and canister is leaking or discharging gas then you should replace that filter you bought with a proper OEM filter. Buy it from Germany auto supply, PartsGeek or your friendly Mercedes dealer just ask for a fuel filter for a 2003 SLK 320. I gave you the part number in a previous post.