Setting Up the Cistern Drain Valve 3/6 L CERSANIT K99-0014: Issues with Water Drainage

According to the instructions, it is regulated by a float on the valve. No matter if I place it at the bottom or at the top, all the water goes down to zero every time. Maybe setting something else affects the correct operation of the cistern?

And it has to go down to "zero". The water level, i.e. the amount of water that is to "go to zero", is set by the height of the float. This will be a certain number of liters in the tank to be drained.
Let me tell you right away that there is no reason to be fooled by the advertising about "only 6 liters" of water when flushing, because it turns out that a good flush requires 3x6 liters of water.
A good cistern is at least 10 liters and suspended at a height of 1.5÷2 meters, because not only the amount of water is important, but also its momentum depending on the height. The higher the altitude, the less water can be used.

No, it`s not supposed to go down to zero. There are two buttons on the cistern, one for draining the entire cistern of water, the other for draining a small amount of water. I repeat the question, how to adjust the flush to dispense little water when pressing the small button? Currently, no matter which one I press, the cistern empties completely. I found the instruction manual for this drain valve, which says that this adjustment is made with a float on the side of the drain valve. In practice, no matter what position I set it in, the cistern empties completely.

I`m not entirely sure, but do you regulate that blue float? I think you have to regulate it with the one on the other side. Try changing the height of the installation on this plastic ladder. It`s probably connected and works with a small button. You can check which button works.

Check if this flush button is installed correctly. Below it you have two mechanisms and the silver buttons must match the position of the mechanism underneath. If you put it the other way around (180°), each button will press the full flush mechanism on a larger surface. If I`m not mistaken, the economical button releases water as long as we keep the button pressed, and when we release the button, the water will stop flowing.

@saskia you were close In my case, it doesn`t matter how the buttons are positioned, because the pin from the button mechanism slides out, when you press the large pin, the pin comes out all the way, saving half a cm less. I finally managed to figure it out through trial and error until the economical button drains the water as long as you hold it. I got used to the previous flush, in which everything happened on the principle of "press and release", I didn`t have to hold the small button. Topic solved.