Secondary air pressure issues

I pull the drain out of the tank and plumb in a gauge.
Just to make sure the gauge on the dash is accurate.

 

Secondary is your front brakes and is all your accessory air circuits such as air suspension, air seats, air horn, 5th wheel slide and so on. Get the air as high as you can, shut down the truck and search for leak. Should be able to hear it. Pay attention to behind the dash, firewall pass through and air tanks.

 

Get a spray bottle, and fill it with soapy water and start spraying everything you can see. Air leaks will cause bubbles to form. Check all your air ride bags and cab suspension bags too, they like to rot and start to leak.

 

Look at all your air bags. If possible, place a jack under the frame and take pressure off of them. They like to get holes between the bottom of the rubber and the base. Spray with soapy water or windex. Cab bags too. Sometimes if the hole is in the right place it will only leak as you are bouncing down the road. Also, the check valve going in to the tank could be buggered and letting air back through the system

 

What he said. Lol

 

I have a question about this. I'm having similar issues but only occasionally. On an '02 9400i, which gauge is the secondary? What is meant by the term "wet tank", and which gauge would correspond to it. And under the truck, which tank is which?