What would cause the pressure to fluctuate in the #2 tank as I am driving down the road (110-125psi). Air dryer is purging every 30-45 secs then pressure drops & builds.
Only while driving, not while spring brakes applied, no audible air leaks when shut down. Gauge only fluctuates while driving.
2001 Nav 9400i.
Was told the air dryer might need replacing or the governor.

That would be my guess - either the air dryer or the compressor governor.

I've had trucks that have done that before - they seem to get "stuck" in a loop, and keep building up air, dumping it, and building it up again. Every time, it was either the dryer or the governor.

rebuild your air dryer ...you should rebuild it every oct or nov of you run cold weather

I've had the purge valve do that, right on the bottom of the air drier. I agree it's the time of year to replace the cartrige in your drier. The oil your compresor makes will plug the filter when it gets cold. (spell check is not working) :lol:

Every second year is lots IMO, unless you live in an area with unusually high humidity.

Use a spray bottle with diluted liquid soap (or leak finder)... check for leaks ; a leaking reservoir signal line will cause compressor cycling... P.S. - this condition is DEATH for your air compressor... it's not designed for such a high duty cycle.

Windex makes really good bubbles. :lol:

Just another suggestion. Could be the clutch fan leaking air. My bosses truck had same problem. Shut truck off and in two minutes all air was gone out of system.

Bingo...I was told by the Shop Foreman that the line to the Fan T-stat was leaking. I had no idea that the air system had anything to do with the fan! I am a car hobbiest...so in my world the fan is powered electrically or by the engine using a thermo clutch.
I never looked under the hood for the problem, I have a garage to do that(I'd get in trouble for messing around with repairs...aka not qualified :lol I was just curious to know as the shop guys I talked to on the phone 2x thru my dispatch gave me vague answers as to the problem but told me it was safe to drive. I had to trust them, but in my eyes I wonder if I should have parked the truck (from air brake course --> air pressure loss over 3psi/min for tractor unit) :? Would "Johnny Law" have nailed me here?