Coming from the BMW world - I would say you speak like a true MB purist......

The system is just a mineral oil hydraulic system - a pump with ram jscks. There is nothing special about it except the poor quality from MB. Why do we always think that because the Germans made it it has to be "special". I agree they do make some very well engineered products, including their cars. They also make some inferior products as well - like their plastic cooling parts, their older AC systems and a lot of their hydraulic systems must be added. I say this because of my experience with BMW power steering, cooling systems and now the MB top mechanisms - they all seem to leak!!!

I would not use most stop leak products in cooling systems but I have used Bars products in an emergency - never had a problem. I also used their hydraulic stop leak in a new floor jack. I bought the jack, left it in the box for 18 months while moving, when I unpacked and put it into service it immediately started to leak. I was upset but not having recourse I decided to try the Bars' stop leak product. It has now been over 6 months, the leak has stopped and I have no problems. It does not work on a principle of "plugging" the leak, it simply conditions the old seals that may have become hard, brittle or otherwise causing the leak to occur. If the seal does not respond, it does cannot help the problem and you simply have to replace the seal go forward.

I have an SLK hardtop (1998) but I can't get the hardtop to release... although the windows go down, the trunk opens, I don't hear the usual release of the hardtop. It was working earlier today..

Remove the small panel on passenger side of trunk - check the fluid level in the reservoir. I would bet it is low, not between the level marks on the side of the round tank.

When it is low there is a good chance it cannot complete the cycle and will present your symptoms.

Good luck.