Question about air valve function /Volvo 780

This 2 air valves are connected ,located inside the frame on the right side; behind the dpf ,under cab,; whats their function,plz ? Brakes ABS only? I've been told by volvo are for cab stability also during high wind or sharp turns ; is this true ?

http://i.imgur.com/EWJeeAO.jpg

 

Thx!
I replaced and this one yesterday in my fury, aiming to fix my bumpy suspension CB antenna is next suspect
So, this doesn't have anything to do with suspension air pressure/ ride comfort ;hmm, 700$ wasted (lots of beer) on parts and 3-4 hours crawling under the truck banging my fingers .
I'm desperately looking to fix my rough ride, I have a Hendrickson air steer axle , I'm just throwing parts/ $$ and I can't get it to ride the way it used to; I replaced air springs, the leaf springs , kingpin bushings , engine mounts,hubs, cab mount, cab shocks/air springs, leveling valves, steer shocks every month ( I tried 3 brands)and still banging bad; Ive been in many Volvo shops, spring shops , I have the pdf manufacturer high settings in my phone, I adjust height but still same, my back hurts. The only thing I noticed ( my gauges are displaying steer /rear axle air pressure ) , before used to maintain 40-45 PSI with same weight on steer, now around 55-65, ive seen 70 psi; This mean more weight is sitting on air spring than leaf springs for some reason; I purchased an adjustable air pressure regulator with gauge, and I'm thinking to install it on air line going in and limit the pressure to 45-50 psi , but I doubt is a good idea.. . Many are blaming the valves, I don't know, drives me nuts, was good comfortable, saved my steer tires for years but I can't have it set up right anymore.

 

Benrab, this is an old thread (2016), i sold the truck in 2017 and got a used 2010/ 780 with Arwin Meritor , leaf spring shock only, less troubles with leveling valves, air leaks .
I used to have heavy duty steer axle 13200 lbs with large air springs. I understand the light one 12,000lbs comes with small air bags , higher pressure and some drivers are saying is more rough.
Here a photo I still have on my phone, this was the 2nd axle, the original was bent during tow with loaded trailer , the tow truck drivers didn't know about special procedures and tools needed for trucks with Hendrickson steertek . Well, can't blame the brave tow truck driver who saved my frozen bacon, i paid him a lunch. I was frozen on Transcanada Hwy near Edmonton, my belts snapped at -46 degrees Celsius; Monday morning December 14 / 2009, Edmonton was the coldest place on Earth. I spent 1000$ for Roadking junk shocks, was still spilling my coffee allover dash .
The whole axle assembly was cheap, about 2000$, makes more sense than doing king pins only or other axle repairs. I ordered through Volvo dealers and takes about 2 weeks to get it because they manufacture it. Arwin Merrittor doesn't offer whole steer axle assembly like this, have to buy piece by piece, the beam only is about 1000$.