On this website, there are some good diagrams and description of what lies behind that small hole, and its connection to the return pipe.
http://www.citroen-ds-id.com/index.html ... pter4.html
Not something I've had to open up, although I have done a new rubber overhaul to one of these.
The same height corrector (for front and rear) were used by Citroen for all of its hydraulic suspension models, up to XM and Xantia. Only the attached bracketry and linkages varied between models.
Our very own
CitroJim produced a really well illustrated guide to overhauling the HC's...
http://www.eastment.net/hcrefurb/index.htm