Pete 379 won't build air....

First off - sorry guys, I am not a trucker, but I am in charge of keeping the trucks on the road for my company.

I have an 07 Pete 379 (CAT engine) with right at 50k miles (local truck only) that will not build air. We replaced the dryer and some hoses back in December because of a similar problem (diagnosed through the local Peterbilt dealership). Now it seems that they just put a bandaid on the problem...

We have tried bypassing the the dryer and it will only build to about 10 psi. Local dealership not much help because we refuse to tow it for another $2000 charge for a problem that should have already been fixed - literally less than 50 miles since problem was "fixed".

We also tried the "hand over the hose" test. We can not get enough pressure to blow your finger off the end of the hose. This tells me it is going to be associated to the compressor in some way, shape or form. I am thinking regulator or bad compressor all together.

Any advice from those who know these trucks better than I? Any advice would be greatly appreciated.

 

Thank you for the replaies...

The governor, from the best I can tell, is mounted to the side of the dryer on this truck. The governor was replaced when the dryer was replaced in December (just found this out after climbing up underneath it). So, I am thinking the unloader - as mentioned above. How would I go about checking this or is this a shop-required item?

Also, I read, just in my internet research, that there could be a reed problem and a rebuild kit is available. Is this true for this truck? If so, would it be worth going this route for a fix? Was told rebuild kit runs about $60

(Cat C15 and Bendix air system)

Honestly, shying away from broken valve in compressor as that sounds like a replacement compressor, to me, and would like to save that as a last resort if possible.

 

UPDATE:
I checked the unloader piston and it seemed OK for the most part. There was a little bit of gumminess but not an excessive amount.
One thing to note: the gasket at the cover was torn. This could possibly be a small part of the issue.

In any case, I am ordering the unloader rebuild/repair kit as it would be cheep to try this out first.

 

Tested that yesterday. The line gets warm to the touch, but not what I would call "hot"...
We bypassed the dryer, as well, and could only get about 10 psi air to register. IMO, we eliminated the dryer from being the issue.
So, I am trying the cheep fix of the unloader piston, then the reed set. If that doesn't repair, then I will suggest replacing the compressor as it may be something more internal. So, I will waste $100 on trying to fix it vs. spending $600 to find out I could have fixed for $100...make sense?

 

Air governor is brand new - was installed in December when dryer was replaced. The truck has been driven about 50 miles since then