[platii]

@flacoramos, since no one answered your concern:

1. Thoroughly clean fluid reservoir and its immediate surroundings
2. Check and correct fill level
   Note: The fill level can come to rest above the ”MAX” mark when the engine is at normal operating temperature. This is dictated by the design in that the marking on the dipstick is referred to a fluid temperature of 20 °C. With the engine at normal operating temperature (approx. 50-60 °C fluid temperature) adjust a fill height 10 mm above the ”MAX” mark. Do not under any circumstances draw off the fluid to the ”MAX” mark when the engine is at normal operating temperature.
3. Start engine.
4. Turn steering wheel left and right twice in each case up to full lock; if necessary, top up hydraulic fluid (e.g. if hydraulic system is completely drained)
5. Move steering wheel to straight-ahead position and turn off engine
6. Check and correct fill level with engine stopped
7. Check hydraulic system for leaks

 

[platii]

dipstick has two marks with no explanation to what's what

That question looks so easy to the point would make the whole thing more complicated, any how the top mark is MAX and the bottom is MIN.
Understanding the check/fill fluid level is part of the answer of your concern, I would assume.
At 20°C fluid temperature you add fluid to the MAX line with engine stopped.
After engine reach operating temperature fluid temp at 50-60°C fluid level should rise to 10mm above the MAX level (engine stopped). This mean the fluid level shouldn't rise more than that unless there is air in the system or the reservoir lid seal is worn.

The reservoir lid seal prone to fail, may be it is your issue if the fluid level is adjusted properly.