My tech's girlfriend has a '92 SL500 and got a chance to look at the 'tubes' when he pulled the cover off. Nice metal tubes(!) with spring loaded pessure relief.
BTW, the
Auto-RX is a great product for cleaning up any engein especially mine with 100k+ miles of engine sludge.
Amazing stuff: it's made of all natural products (water soluble) and it slowly cleans the engine.
After removing the left side valve cover last week, the engine looks like it's new! No kidding.
Check out the site:
http://www.auto-rx.com/