Citroen Xantia 1.9 TD Estate suspension problem?

[Dexy]

Citroen experts only please, or someone who's had the same problem. The suspension height 'regulator' fails to recognise the 'normal ride' setting. Low setting is fine, off road setting is fine. It will go from low to high, and high to low and that's it. It won't go from low to normal, or high to normal. What does the accumulator sphere do, and is the anti-sink valve the same thing as the accumulator sphere. Is there an accumulator sphere on the front as well as the back, and if it is a different thing, does it have a anti-sink valve on both the front and the rear. Is the LHM fluid level crucial to this problem and how do I get the level between the two red rings on the glass dome on top of the reservoir? I've never seen the level between the two red rings no matter what I do! Also, the front end occasionally sinks noisily when I turn the engine off (maybe once in 20 times I switch off) Citroen suspensions a problem....I should say so! I'm out of space, so more in next question