XJ40 Problem with hydraulic system, Brake and leveling

I hope that the Google translation is somewhat readable.

I have the following problems:

  • No braking power
  • On the lid of the green reservoir fluid comes out when releasing the brake.
    After a few brake applications the reservoir is almost empty.

After replacing the black ball, the brake pressure is back to normal.
The ABS is from BOSCH 0 265 200 022

I hope someone can give me a hint where to look for the error.

Model: XJ40 Sovereign 3.6i 1988

Many thanks from Switzerland