I have replaced an A/C hose on my S320. This hose is part tubing/hose that rest on the left fender. The bottom of the tubing was rusted out, you will need to pull out the insulation to view the damage tubing. This is an expansive hose, a shop that repair hydraulic hose replaced the rust part for less than $25.

If your have leak due to rust like mine, you will need to replace the dyer.

Jack