Air Pressure problem

With the engine running, put your hand under the air dryer purge valve to know if air is coming out of it all the time. Mine had a leak through the purge valve, although eventually I found out the problem was a bad air compressor leaking oil through the main air hose and making its way to the air dryer. Maybe all you need is to change the purge valve and the air filter.

 

Probably a hose to the “new” air dryer. Always start where the last repairs were made.
There is a hose that goes from air tank to air governor that tells it when cut in and out which tell the air dryer to purge.
Soapy water is your friend.

 

Fan hub

 

My w900 does the same thing.

 

If no leaks, my bet is someone used the wrong air dryer. I did this and had to add an extra valve. The dryer looked the same but compressor would run constant, like yours.