rjk,
If you're sure it's hydraulic from your selfleveling system it needs to be replaced by the MB ZH-M or the Febi ZH-M (1 litre part no. 02615). The Febi brand is available at MB parts suppliers like Randy Steele at
www.epsparts.com or a local indy that specializes in MB's. The dealer will have the MB brand ZH-M. The system holds a little over 2 litres as I recall.
Obviously something's come loose or a hose has failed. The power steering pump and the self leveling system use the same pump. Actually, it's one pump with two seperate vanes and compartments for the whirling works. The forward section houses the PS fluid and vanes; the rear has the ZH-M and impellers for the S-L system.
As one member suggested, clean off the motor, drop the lower engine panel and see where it leaks. You DO NOT want to run this system dry. Bad things happen to PS pumps when there is no fluid to lube them. Same goes with the rear of the pump. Depending on how severe the leak is you may want to have the wagon flat bedded to a shop. Cheaper in the long run.
May I humbly suggest that you buy a Service Manual CD for your wagon. It makes for a stronger relationship between you and your car and you and your tech.
Good Luck,
Hazen
91 300TE 4Matic
133k mi.