Loosing air in secondary tank.

Not too hard,if you can get a extra set of hands it will be a lot easier to get off and on.
Is the check engine light on?
Need to find out which fan clutch you have,some operate with air on,some air off.
Can you spin the fan by hand with motor off key off? Key on?

 

Submerge the truck under water and look for the bubbles..

 

Not sure but I had a similar problem with my Mack on the primary side. There is a value that pop's off when full, I took it apart and found the O RING was wore out, got a new one from Lowe's.

 

You can get a rebuild kit for most hubs. www.kitmasters.com. It's not a hard job to rebuild, you'll need snap ring pliers, a torque wrench (with the correct size socket, 2" is popular), a press to push the bearings out/in and probably a torx depending on what hub you have, T27 is common.

Getting the hub off and back on is the hardest part. They are heavy and awkward, they can easily puncture the radiator if you're not careful and most trucks have little room to get them out. It helps to used the belt that you take off it around the pulley and sit up on the engine to lift it while someone else guides it out. I also tape a piece of cardboard to the radiator, so I don't damage it if I bump it.

 

If the the air fan clutch solenoid is setup to be closed when de-energized, you wouldn't hear air leaking through the clutch when the truck was turned off. It's closed when you kill the engine. I just went through this with my Cat and it's solenoid is setup to be open when de-energized. In my case you kill the motor and mine would start leaking air big time. The air would leak out completely in about 5 minutes or less. It was ok running because 95% of the time the fan wasn't being called for so the solenoid was closed.


oops, may be to late.........

 

And when the engine is running the govenor is building up your air pressure..

 

Need to idle truck so your losing air,check the line coming from fan,trace it back to solenoid,check solenoid also. It would suck to replace fan and have same problem.
Compare price of a rebuild kit versus a reman,may not be a big difference.

 

The best way to check for a fan hub leak is with the engine off so you can hear. Unplug the solenoid and jump power/ground to activate it. That will put air to the hub and you can hear the leak without the engine running. If you're lucky, it's just a hose or fitting. If the hub itself is leaking, rebuild or replace. A leaking hub may just be a seal, but more often than not, heat form a bearing going out ruined the seals. A bearing failure could put your fan in to the radiator, so it is not something that needs to be checked ASAP.