I highly recommend having the leak fixed as soon as possible before the suspension fails. The reservoir is under the hood, on the driver's side, in front of the shock tower. It is black with a small rubber cap/dipstick and should be labeled ABC. Also, make sure that you use Mercedes-Benz approved synthetic hydraulic fluid to prevent any damage or contamination to the system.
Looking at the engine, it is on the right side black and round close to headlight. The fluid is special and expensive! Clean area before removing dipstick !!! Use brake cleaner spray if you have it. It does not leave residue. Any particle of dirt can destroy the tandem pump! ($1800) Napa sells the only alternative hydraulic fluid that will work other than MB fluid (over $50 a liter), it is called Pentosin CHF- 11S in a green rectangular liter can. Around $18-$20 a liter. You may want to change the filter also ($40) Good luck!!!
Thank you swordtwo, my 2005 SL500 just dumped a 3/4 cup of fluid from the ABC overnight. So at least I have a starting point. Had two accumulators replaced 3 weeks ago. I'm thinking there may be some air in the system.
You need the Mercedes Star system computer and run a RODEO which is very entertaining to see. Like a Lowrider convention in LA! See it on YouTube. Cannot do this without computer, but you can manually raise and lower car which may help.