Air not Building on a Freightliner Cascadia

Hello, new to this forum. I have a concern with a 2015 Freightliner Cascadia. One of my Techs R&R the Air Drier w/ Air Governor and R&R the tank that the Air Drier mounts to. He thought there was a blockage inside the tank since its a 2 part tank. The truck isn't building Primary Air at all and building about 40-50 psi on the secondary. Air is coming out of the Air compressor > Air Drier and is building air in the wet tank portion of the air tank. ( The side of the tank that the Air Drier is mounted too. ) I'm not getting any air out of the green supply line from the Air Drier to the Primary Tank. What could be the problem. I pray it's not the unloader valve or anything to do with the Air Compressor with it being a pain in the back of a job.

 

* Fixed * Ended up hooking shop air directly to the Air Drier. And the truck builded 125 psi on primary and secondary. Which is telling me the air compressor isn't building air properly. I could've replaced the Unloader Valve on the Air compressor and fixed it but with the age of the compressor, we decided to replace the Air Compressor. ( Which comes with the new Unloader Valve. )