WELCOME to this forum and Citroens too - there may be no going back!!
You would do a lot worse than read this link's information though it does
go into a great deal of detail and is specifically for the Citroen Bx but the
inherent design is the same on the Xantia:
http://www.tramontana.co.hu/citroen/sus ... ension.php
As you're new to Citroen hydraulics any work can only be undertaken
when you release the pressure - it is quite considerable at 1500-2000psi
so care must be taken and a decent 'respect' is required!!
The Xantia can be depressurised initially by selecting the height adjuster
lever on the central console TO IT'S LOWEST SETTING situated/marked
beside the handbrake lever. Ultimately a reserve of pressure is required
to be released on the main hydraulic pressure 'switch' or regulator -
located on the body upon which another sphere is attached on the front
of the engine. A simple looking 12mm bolt head is the release or so called
'bleed' screw which MUST NOT be totally removed but turned about
90-100degrees until a sound of whooshing air is heard.
Great care needs to be exercised with space allowed around/under the car
as it will drop right down leaving mere inches of space underneath unlike
any other car but once the height has dropped it is stable and safe to work
on taking into account travel of arms/wishbones will have flaccid
The system itself is self bleeding and that is what the aforementioned
12mm release screw is for with the engine running. In essence the
maximum high setting is generally used as a reference as this is
maximum presssure where all the systems fluid is being sent throughout
the suspension to lift it up and the reservoir of fluid is checked for its
correct minimum level by the float on its top.
There's a whole forumsworth of keen DIY'er Cit nuts here so you may
even find there's one in a garage/driveway near you now so help advice
or just moral support could be the answer if you get a bit confounded by
the Citroeny extra bits!!