Have a 2003 SLK230 6 speed. Clutch pedal slow to come up sometimes. Looked under the dash for carpet or mats causing it & noticed the spring on the pedal is stretched where only the last 2 links stretch when pushing pedal down. Is this normal, asked the dealer while my other MB in for service but they did not know even though I showed a pictre of it because they said they don't service many manual trans but lead tech took the picture to check it out.

 

The clutch pedal for most of the MB cars has hydraulic support (not only spring-type). Check the connection and also for leaks around the pedal. If I'm not completely mistaken it is coming from the brakes system.

 

The hydraulic system for the clutch is separate from the brakes.

I'm not certain but it sounds like the return spring you mention has been stretched.