Secondary air pressure keeps dropping/fan turns kn

Well it just got really bad had to pull over, secondary air goes all the down below 50, and engine fan is on non stop,
not sure what to do :/

 

Could it be something with the air tanks? Air dryer? I leaked all the air from the tanks and was able to drive for about an hour before 2ndry air staryed going down again. Something about the air not releasing when I drive.
Air psi slowly goes down when im driving but something’s not doing its job to bring it back up? Moisture building up in tanks?

 

Im leaning towards the air dryer, its not purging when it has to, or something with the secondary air tank

 

Went to Peterbilt in Council Bluffs IA, they looked at the air dryer and seen no problem. Then they inspected for leaks and decided to change the hill assist air valve. They charged me $900 at the end, and then I start driving - not even an hour in - same thing starts happening.
Had to pull over 4 times to drain air tanks so I coulve made it home.

I noticed a pattern:
1. Drain air tanks
2. Start driving and everything is normal
3. PSI of both go up to 120-125 and then slowlyyy drop down go 110
4. Truck releases air and it goes back to 120-125
5. That happens for some time - everything is normal, air is being purged, psi goes back to normal, until:
6. Truck doesn't want to purge air
anymore, its not engaging anymore
7. Engine fan turns on/secondary air goes all the way down


So now I have no idea what the real issue is, any ideas? Electrical? Bad relay?

 

Well now I have 2 mechanics telling me its an electrical issue.
What it is idk, he recommended that I try replacing the secondary air valve sensor that sits behind the dash. It's $210, so idk. He unplugged and plugged the sensor a few times and it seemed to work just fine so not sure.
Maybe it malfunctions as I drive and the truck rattling a little makes it act up.
Even so it doesn’t explain why the truck stops purging air after a while.

 

If anyone’s having similar issue:
A local mechanic unplugged and plugged the secondary air gauge sensor (that sits behind the middle of the dash) and problem was fixed. I have no idea how it worked but it did, ever since that no problems.
Im guessing the sensor got loose somehow and was causing an electrical issue that triggered the secondary air gauge to randomly go down nonstop.
Only culprit I can think of is I did use the trailer brake occasionally and heard air-like noise behind the dash, maybe that had some play in it