the car works like new ;o)
actually, it seems the problem was a bit complex.
i've filled the pump and the pipe with lhm, but still seen no change in its behaviour. so i decided to leave the car running for a while, and when i returned i saw the wheels standing in normal height. so finally it started to work. i've done citrobatics for like 30 minutes or so, depressurizing the system 4-5 times, still lots of air, and less and less lhm in the reservoir. so i started to investigate what else could be the problem and noticed a large spill of lhm under the car (hardly noticeable as we had a rainy weather these days), and also noticed lhm dripping from somewhere above. first i suspected the lhm reservoir, but then i noticed that one rubber pipe got broken in the wheel bell. i removed the old part of the pipe, plug it back, and now everything works fine.
i will do some test drive now to see how suspension works, but it already looks much more comfortable pushing the car by hands than it was in the last days. (i cleaned the filters originally because i felt that the suspension is too hard).
afterall, now i know that the source of the whole problem was a leak where the car lost hydraulic pressure and took up some air. too bad that the pipe that got aged couldn't be seen because in normal and lowest setting it's covered by the wheel.
so thanks again for your help!
while bugging with all these, i had to notice, that actually a citroën isn't complicated at all. it's just a bit different.
)