I have a 91 Mecedez with 153,000 miles thats terrific but the mystery leak in the hydraulic running the rear suspension is driving me nuts. Especially when the hydraulics call for factory spec fluid. Any ideas?

Clean well first.
All the lines, fittings, pumps, everything. An engine degreaser or brake & parts cleaner from an auto parts store will cut the greasy oily build up.
When it’s clean you can quickly see a leak. ( unless a pump is leaking internally )
On a car that age it’s probably just an o-ring or two but where ?

Clean first for easiest chance to find it.

Thanks Ken, will do

Was this suspension system an option? If so, it might be cheaper just to replace it with the standard springs, which never leak out like oil and air tend to do…