Air compressor keeps building

That sounds like the unloader piston is stuck or the passage for the governor control pressure to the piston is blocked. I would first be sure the governor is working; the unloader needs at least 60 psi to operate. So measure the pressure at the port to see if you have that pressure available. A simple test of the unloader would be to connect shop air to the governor port using a regulator and see if the compressor unloads when you apply 65-70 PSI. If it does the governor is the issue, if it doesn't the unloader passage or piston is the problem. For both I would say to swap the defective unit as they are new.

Air drier shouldn't make a difference other than if you had one you set the governor up, so it shuts the compressor off after the drier dumps normally. The way the system works is somewhat simple. Compressor pulls in air, compresses it and sends it out to the tank and governor, the governor acts like a pressure regulator but instead of just keeping the pressure constant the governor hits the set pressure and opens the port to the compressor that causes the unloader piston to push down on the unloader valve and now the air just bounces from one cylinder to the other until the pressure drops and the port closes and the compressor pumps again. You could remove the line going to the unloader port and see if the governor is working, when it hits the set pressure that line will have high flow air, the rest of the time it will normally have no pressure.