Air pressure gauges not indicating evenly

[09pornstar]

My prostar started developing another beautiful sympthom.
When i press the brake pedal only #1 gauge goes down, the other stays at 120 the whole time. When i get off the highway it slowly goes to 90 just from one pump. No audible air leaks whatsoever on tractor or trailer.
Im bringing the truck in this weekend to get the camshafts replaced but i would like to know what to expect and maybe order parts. Thanks!