Cat 6NZ air compressor won't shut off

Truck builds air pressure fine to 120lbs, the dryer purges then hisses for a few seconds, then it quits hissing & purges again & keeps repeating this cycle. Through all of this the gauge never moves, stays right at 120lbs. Thought it might be the air governor so I replaced it & it didn't change anything. Thought it might be the purge valve on the dryer so I replaced that & it didn't help. Thought it might be the unloader valves in the head of the compressor so I replaced those and that didn't change anything either. Any ideas what it could be?

 

Sprayed all air lines, tanks and fittings down with soapy water, found a few small leaks at some of the fittings but nothing major. Still hissing and purging even after I tightened the fittings up. Went to the parts store and got a new check valve for the AD9 dryer and everything is working as it should now.