Freightliner has a lot of info on their web site. Look at the link below:
XC-S Chassis
It is sometimes hard to figure out what was installed by freightliner and what was done by the body builder. For example, your air dump valve does not look like the one on my holiday ramber/freightliner, but accomplishes the same goal.
If you really want to understand the freightliner chassis, check out the "Camp Freightliner". It's a 2 day class taught in Gaffney, SC. There are many postings about camp freightliner.
Fred