The air compressor has something called "unloader valves." The air from the regulator on the firewall opens these valves and makes the compressor not compress. When the regulator decides the system needs more air, it removes the air from the valves and the compressor starts pumping again.
Start simple. Remove the intake screen/filter on the compressor and make sure it isn't plugged up. Still no workie, then start looking at the unloader valves. They are basically held down by a spring. Pull the cover off and check out the springs. My Gasser had one of the two springs broken off just a little at the end of the coil. I put a washer in as a shim and have had 110 psi air ever since.
If the unloaders check out ok, then you need to start thinking about rings. However, first pull the output line from the compressor, fire up the truck and see if it is pumping air now. You could have a plugged up line somewhere between the compressor and the tanks. No air out means no air in.
Let us know what you find.