Loss of air on 2013 cascadia DD 15 while driving?

While driving my truck loses air pressure at a rate of approx every 4 and a half minutes. Both air gauges go down then compressor kicks in and air dryer blows off when back up to normal air pressure. Does not occur when sitting and idling.

 

I have had the truck since new. This ordeal started a couple of months ago. I put a new governor and air dryer on it. Also needed a new fan hub assembly, so we know it isn't that. The air bags are good so yes we are thinking it might be the DEF system. My DEF system does use air. Gonna have to check the air pump.

 

I will have to check that out. Never thought of that.