Loosing air in secondary tank.

Do you have a fan switch? If so turn it on see if that stops the leak.
If the unloader was sticking I think you would be loosing on the primary side.

 

couldn't find the air leak in my 06 9400, there is a baffle in the one tank , was loosing secondary air as well,
a pin hole worked it's way through from the weld on the inside around the baffle, power washed the other one
and chunks started to fall off it, just like a Navy ship held together by paint

 

Turn engine on,build air until the govenor cuts off,put truck in 1st gear,shut off truck,turn key to on position without starting,push in red valve,yellow valve out,get out walk around and listen for air leak

 

Ok then, leave trk running,put in neutral,chalk wheels,push in both air valves,then get out and listen for air,you may be able to hear air even with engine on if you applied both valves.

 

Only thing I can think of is the seal in the fan clutch. That's why I said turn the fan on and see if the leak stops.