i bought this so called service manual and it is very limited on the 4 matic and any hydraulic problems , my guess would be a power steering pump is probably blown a seal and it is leaking into the rear pump which is where your hydrualic pump is , the pump is dual purpose and has a seperate reservoir for p.s and if the fluid is going from the reservoir th the hydraulic it is most likely the cause. i have a 4 matic and it also uses the same type of pump look into the pump funtions in the manual