Air pressure gauges not indicating evenly

[bwake]

(10-15-2018 )09pornstar Wrote:  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!

When you press the pedal and the #1 gauge is the only one to drop, how hard are you getting on the brake pedal? The #1 or primary system is what controls the drive axle brakes and the steer axle is controlled by the #2 or secondary reservoir. If you are only lightly getting on the pedal, there is no reason that the steer axles are going to engage if you are loaded. (weight on drives so that's where we want the braking to happen). I would check the applied stroke with full brake application on all 6 brake positions and depending on what size chambers you are running, the stroke should be around 1 1/4 to 1 1/2". Your max stroke (again, depends on the chamber size and standard or long stroke) will be no more than 2 to 2 1/2". If for some reason any position has no stroke (possibly steer based on the complaint) you are going to have to look into the treadle valve (brake valve), the line going from it to the quick release valve under the radiator (usually) to make sure that you have psi going there. If so, check the air lines from the QR valve to each steer chamber.
One thing that many don't understand on brake stroke is this: if you have a wheel end that is out of specs on the stroke, there is an issue with the foundation brakes that needs to be addressed! Auto slacks are made to keep these things in proper adjustment and if it can't, there is a problem you need to find and fix. Do not just adjust it and hope the issue goes away, it will come back when the brakes wear again leaving you in the same position you were in to begin with.
When you say "it" goes to 90. Are we talking only one gauge, both gauges, happens every time or only in certain situations, etc? Depending on which one this is happening to can maybe tell us a little more about what is happening. The secondary system runs a lot of stuff on these trucks (fan clutch, air ride suspension (cab, chassis and seat), supplying air to the trailer tanks if the double check valve is working correctly.
There are so many things that are not going to be easy to diagnose over the web and I could write a book on what to look for depending on how the system is operating. Attached is a link to a basic air system that may help some.

http://www.totaltruckparts.net/brake/airschematics.html