If the check valve is leaking, you should feel constant air comeing out of the dryer when the compressor un-loads, until it starts compressing again, since you didn't mention it was doing this, I assume the check isn't the problem. Normally when it "blows off" it will vent for a few seconds then stop. a good air system should run 15 miniutes or so between cycles while running down the highway (no brakes applications or other air uses) a really good system will do better. My 9670 use to go 45 miniutes between cycles! I am parked next to a truck doing the same thing... its bothersome.
If the un loader is leaking, it will will also cause a loss of air in the wet tank, as it uses that to sense and unload. As Glenn said, put a guage in the "Res" line on the govenor and see what is happening. I have seen braided line that looked ok from the outside, but leaked like a sieve when sprayed with soapy water.
you can temporarrly plug the unloader line to the air dryer and see if that stops it short cycleing, if so then the problem is at the dryer, if not then look elsewhere.
Oh, and since both guages don't show a loss, (because the primary and secondary tanks both have check valves (which are working) between them and the wet tank) you know the problem isn't down stream from the wet tank.
Many drivers see both gauges up and think their system is working fine... when actually it is in need of service. You should see it biuld and then fall back to 90 psi or so as the brakes are used, then biuld again to cut-out (120-125 psi). If it stays all the time at 120 (esp if you here it blow off every few sec) it is not funtioning properly.