Problem Building Air Pressure when cold

Also under Fifthwheel is ATF valve (abs module) these are real famous for leaking as well and exhaust line from left side of fire wall another famous if line leaking dash valves are leaking by one another

 

Also check for two valves (one red and one blue) near your air dryer. The '08 Pete I was driving suddenly wouldn't build air pressure. We checked that truck over and over then the mechanic noticed these two knobs. He turned them both to the right until they got tight then backed them off and it started building air pressure. Anybody know what those valves are for?

 

The unloader valve on the Holset single cylinder compressors are famous for this. I have seen many repeat failures of this even at low mileage. An unloader kit and a good cleaning should correct the problem.

 

I know this is an old thread, but I'm having the exact same issue with my 2010 Prostar with ISX871. Every morning now it would build pressure when first started, but after I get going it will start losing pressure and not build at all until engine is shut off and started again or until engine warms up. Once it is warm it builds air normal.
dmateiovici , can you please confirm that the issue was the compressor unloader valve, and if not, what was it?

 

I had this same issue on a BX-2150 compressor on an International bus. Builds air fine the first time, once it cuts out it doesn't build air anymore.

I removed the governor from the compressor. Just loosen the bolts. There was no air leaking and the compressor does not charge. So the governor is not the cause. I removed the intake and exhaust lines to eliminate the possibility for a clog / kink .. and it was still not really pumping .. a little bit but I could easily stop it with my thumb. Probably no more than 5 psi.

Next I actually took the unloader valve completely out of the compressor with the engine running and it still was not building any air. So that proves the unloader was NOT stuck and holding the valve open.

You could look down in the unloader hole and see the intake valve. I poked at it a few times with an Allen wrench and presto, it started building air again. So the intake valve was stuck open all by itself, not being forced.

I could see there is crud on the unloader valve, old oil, tar like. I tried to wipe it off but it was real thick and gummy. So I dumped it in some WD-40 and with that the crud melted right off.

So I filled the compressor with WD-40 , will let it sit overnight. Maybe hit it with some carb cleaner in the morning. Hopefully this will dissolve the crud (and not the seals)...

I've read that caked on oil could be caused by a restriction. I saw a slight kink in the discharge hose but can't really detect a restriction. With the compressor pumping and the far side of the discharge hose disconnected I get a crisp pulsing. I would expect with a restriction it would have more of a constant hiss.

So .. that's how I diagnosed my system .. this post is still coming up in google searches so I thought I'd add my .02
Still TBD whether cleaning the intake valve works or whether I will have to bite the bullet and get a new compressor. Technically it would just be the head but I don't know how feasible it is to replace a head while hanging upside down in a hole in the floor..