I finally got a "thickness,"

The vehicle is first hand and checkbook maintained, BJ 88
The vehicle was about 450km away from me.
On the return trip with the V8 much to me after about 200 km in the traffic jam to which a really bad noise came from the engine compartment.
The noise did not change after speed increased.
The beat was always the same but always all 4Sek.
When I arrived home, I noticed a strange smell of hydraulic oil.
After further sightseeing, many green drops on the ground.
After tracing, I saw the drops coming from the direction of the servo pump.
The level in the container was no longer between min. and max.
I replenished the original oil and that nasty sound was gone too. All lines and connections are tight.
What can this be or are there experiences of you in this case?
Would be very happy about possible help and spare parts seals etc. (where you get them)
Thank you very much.