Here's a link to the site run by the legendary Gabor Deak Jahn. He and Zeljko Nastasic have produced the definitive explanation of the Hydractive systems fitted to Xm's and Xantias. A good site full of useful information for the enthusiast. Although the basic site is dedicated to BX's there is a reasonable section on XM's which is continually growing. Since the XM shares components with other models most of the information applies to more than just the model it was written for.

http://www.tramontana.co.hu/citroen/index.html

cheers

noz