[DK on the road]

Symptoms;
Engine compressor causing Air dryer to constantly blow off after it hits 120psi. pst,pst,pst,pst. very rapidly.
it builds pressure fine and holds pressure, but once to 120 (+/-) it continually releases until the engine is shut down.

How to diagnose.
On mine, the D2 governor is located behind the air dryer, inboard of the left rear tag wheel. it has a 2 lines 1/4" lines attached.  One line comes from the primary tank, the goes back to the compressor. If you remove the 1/4" line at the compressor at start up, and the chassis has no air built up yet, the line will not have air coming out of it. As the system build pressure and the governor recognizes 120psi, it will start to release air, back into the compressor into the unloading valve. This valve uses air to open the compressors valves so that it, in a sense, free spins or does not build pressure until a pressure drop to 90 ( or what ever your governor is set at) is recognized at the governor. 

Repair
The Unloading valve is on top rear center of my Bendix compressor with 2 small Torx bolts holding it down the cover. Its is qn upper plunger inside of a larger plunger, all with about 4-5 O rings
My unloader valve had a bad O ring and leaking internally therefore it was not moving correctly, therefore, allowing the compressor to run all the time and making (forcing) the air dryer to open its relief/pop off valve to control the pressure.

I went into CAT with my Bendix air compressor part numbers, prepared to get my exact part. I was told, I was better off giving them my engine serial number. For that reason, I do not offer you my part number since it correlates directly to your engine.  About $120 and 1/2 hour and I was back to running normal.

I also watch several good YouTube Video's using the search "unloading valve", how to install, ect.

I'm not an expert, but this is my best understanding. please feel free to add or correct this post.
Hope this helps somebody.