1998 International 9400 air leak

Get a spray bottle, fill with water and a few drops of dish soap. Run the truck until the air dryer releases, shut the truck down and start spraying... Look for tiny bubbles.

 

That's where I would start. But there is no guarantee that's where the problem is. It's an old truck. Could be anywhere. Have a 9200. Seat leaked. Connection at firewall leaked. Fitting under the truck leaked. Airbags leaked. Air tank plugs all leaked. Airline to the brake release knob. Airline to the splitter. Fix one, another will pop up somewhere. Count on it.

 

You won't know til you crawl under there with a spray bottle.

 

You should have check valves on your air tanks that only let the air go in One Direction.

I can't imagine any mechanic ever removing a check valve, but if it sticks open you could have a problem with it running back to the compressor.

 

I know that the primary and secondary tanks have check valves but some trucks have a double tank (wet tank/secondary?) that has a non serviceable check valve in between. I'm not sure if a stand-alone wet tank itself has a check valve or not.

It's best if you get the information from your dealer because then you'll exactly know what's what. They should be able to tell you exactly what check valves are where.

It is always best for you to follow things exactly the way the manufacturer designed them.

 

Start with the air tank that they moved and start spraying the air lines with soapy water, you'll eventually find the leak.