Sorry, I can't see how the brake oil is entering the hydraulic system. I can, however, see, and have seen, it getting into the rear differential oil. Maybe that is what you meant? If that is happening I feel very confident that your brake linings are worn out. When this happens it allows the brake pistons to travel so far that the seals allow brake oil to pass (which is how brake fluid ends up in the rear diff). This has happened 2x on my 416C. Both times the brakes were worn out. I got about 5,000 hours from each set of brakes.