any way to lower the PSI that the air governor calls for air?

Since you replaced almost everything related it may be worth to try replacing that valve assembly. Did a new dryer come with that valve installed?

 

You have to use shop air into the compressor discharge line, with a shut off, and that way you can test things out properly, if air is leaking back into the intake lt is the unloaders if it is leaking out the purge valve it is the purge valve or the cartridge is not sealing off properly, there are different purge valves that need to be tested by pressurizing the intake to be sure it does not lose boost intake air.
It sounds to me it is the cartridge is not sealing off the air from the wet tank.
Just a thought!

 

What engine do you have in it?

For some reason Bendix has a setup on some engines like Cummins that is different then the norm. I had a bunch of issues when I replaced the entire AD-9 unit with a reman one and found out that I didn't have the right "add on" in the air dryer that allowed the governor to work right. When I ended up fixing that problem, I decided to replace the governor and when I did, I just swapped it out with the old one only to find out that one of the ports I used were not the right ones and had to move the port to the right one.