Air compressor problem

Truck is a 2000 FL with Detroit S60 in it. Truck has been sitting for about 2 weeks and I intend to go back to work tomorrow. After having to charge the batteries for a couple hours, finally got it to start. The air compressor gets to about 75 psi, then the rapid fire (every 2 to 5 seconds) sounds like purge valve is opening. The sound is coming from under the hood near the compressor .. the air dryer is back behind the fuel tanks. It will not build beyond 75 psi. The truck has a new air dryer on it - Bendix - and was put on just a few months ago. It hasn't been all that cold for the most part, we did have a few days that went to maybe 0 F, but the average would still be above freezing. Where should I start with this?

 

Got it figured out. After methanol in the air line from compressor it took just a few seconds and it cleared it right up. Never had this problem before. Thanks for the help.