Volvo 2007 670 Air compressor problem

Please Helppp!!! I have a 2007 Volvo 670 with an Volvo D12 engine in it. I have a big issue with my air pressure, my air compressor is constantly running, building air up and it purges air every 30-45 seconds. I have so far replaced my air dryer filter (once in June of this year) and I am replacing it now again because it combusted (has two straight lines through the middle like someone cut it with a knife). I have replaced my shut off valve on the air dryer, and the rubber little gaskets that go in the valve. I have also replaced my firewall wiring that is under the hood on the side where the steering wheel is. Recently i replaced the fitting on the compressor that goes from the compressor into the little blue line, and I have replaced the air governor also. None of these things that I have done made the compressor stop building air, nor made it stop purgeing the air every 30-45 seconds.
Also on a side note since I replaced my air governor my PSI in my air tanks went down from 120 in the tanks to 100 before it purges. Please help I am sick of trying to figure out what is wrong with this darn thing.