Front Air Pressure

Normal pressure for mine (2 years newer) front and back is 120, and that doesn't hardly change unless I'm modulating the brakes.

Thanks - yep Brett, that pressure swing is more accurate - I was going by memory but now that I look at the gauge 105-120 is the right swing. So sounds like they'll likely be the same pressure swing on both from Brian's description - so sounds like I've likely got a bad gauge - I'll try there first, followed by the check valve. Thanks for the direction guys!
- Tony

And, normal cut-in to cut-out is 10-20 PSI. Example cut-in 105 PSI, cut-out 120 PSI.

140 sounds high. Check with Freightliner, Gaffney. Most governors are easily adjustable.

If you don't know what causes the pressure to fluctuate, then you probably shouldn't be working on the air system yourself. Air pressure is supplied by the compressor on the engine, which is controlled by an air governor. As you apply the brakes air is used and exhausted, and when air pressure falls to the governor's cut in pressure it tells the compressor to start compressing air. When pressure is raised to the governor's cut-out pressure it tells the compressor to stop compressing air and also tells the air dryer to purge. If the front air pressure stayed static at 100 psi while the other air pressure gauge rose and fell normally there is something wrong with the air system. Might be a check valve, more probably the gauge itself might be stuck. Definitely need to get that checked out.

Brian