I have a 2007 freightliner century. I have an air leak that I cant find. I have no leak when the engine and the key is off. If I turn the ignition on then I have an air leak. I have done the following

1 new secondary air tank
2 all new air connections on air tank
3 all new check valves and release valves on the secondary tank
4 new fan hub and clutch
5 new air connections on fan hub and fan hub solenoid.

at this point I have no idea were to even look at this point. any ideas?

 

Forgot to mention. Its the secondary tank thats leaking. Primary tank only drops when braking

 

I have had air leaks that were difficult to find. In my case it usually winds up being an airbag. Get a spray bottle and fill it up with a little dishwashing detergent and water. Spray lines, airbag's and fittings. If you see bubbles you will have found your air leak.

 

I had one recently, could hear air but couldn't find it. Shop got under it, found the Compressor was blowing oil out of it and that was the problem of losing air pressure when the truck was sitting turned off during a delivery.

 

gman said it right. I had to do that and found a leak,fixed it and another one sprung up in a week.i ended up fixing a bunch of leaks
my secondary was the problem,under the cab a exhaust leak had heated up some small air lines and they started popping like popcorn.I fixed the exhaust leak too.

 

I had a couple of air bags that were leaking. All of them shown signs of dry rot. I just replaced the last two this week. I now have 8 new airbag's.