M939 Won't build air pressure

[ARYankee]

Well I found my problem today and fixed it. I did take apart the unloader on top of the air compressor and cleaned it. That wasn't the issue. I started it up after doing that and still wasn't building pressure so I said might as well do a quick walk around the truck and listen for air leaks. I got to the passenger side and saw a filter laying on the frame next to some hoses. Upon crawling under the truck it was the air dryer filter. It somehow came loose and fell off. Luckily I didn't completely lose it on the road or anything. I have no idea how I missed that originally when I first encountered my problem. It's always the simple things that get you. I got it put back on and tightened down so it runs good now. All I need to do is fix some of the damage that some rat did to the interior of the truck from sitting for the last 4 months.

 

[Artisan]

I have several first hand reports of undercarriage damage to many trucks coming from
Fontana and Barstow. We assume they are short cutting truck removal from trailers and using
short forks to unload. That can cause BIG damage. One report was specifically
the Air Dryer munched, I was told fork lift fork marks were seen w/o question.

Be afraid.

 

[dilligaf13]

Now that's a pretty inexpensive fix. Glad it was something simple AR.

 

[Kegs35]

I just spent the last 2 days dealing with the same issues on my 931A2. At first it wouldn't build any air at all, so I took Twinturbo's advice & took the top of the compressor off.......he was right, the gasket was so dry rotted it was basically non-existent. Every drop of air it was making leaked out before ever getting into a line. 2 minutes with some gasket material & an Xacto knife & voila........120 psi in no time! Then minutes later came another surprise, air leaking out of the bottom of the dryer at a faster pace than the compressor could keep up with. The unloader valve was stuck wide open & wouldn't close no matter how hard I tried. Had to take the dryer apart & clean out tons of junk & mineral scale. I took the valve apart, cleaned all of the scale out, and applied a bit of o-ring grease to all of the rings & seats. This was about a 20 minute job, and after what I saw in there I am going to make this a part of my standard maintenance procedure when receiving new trucks. Thanks to everyone's input earlier in this thread for pointing me in the right direction for this problem......I was at a complete loss before reading all of your ideas. Hopefully my input here will help someone else further down the line.

 

[mikey]

Thank you ARYankee. I had the same issue with my M931A2, the truck would not build air. The truck was fine on the pretrip inspection. It built air quickly and was venting excess air. I got about ten miles down the road and the low air buzzer came on. The resolution to my problem was also the air dryer filter, although mine was still on the the truck, it was just very very loose. This was tough to troubleshoot because with the truck running, you could not hear the air leaking. With the truck off, there was no leak because the filter is before the air tanks. The tanks held pressure for hours as long as I did not touch the brake. I believe that you saved me hours of searching for the issue. Once I confirmed, via the TM troubleshooting steps, that the compressor was working the filter was the first thing I checked.

Thanks again for posting your resolution.

Mikey

 

[WSm925a2]

I am having the same issue with not building pressure past 30psi, I did find air steadily coming from the air dryer purge valve. Listening to others advice, I replaced the dryer with a brand new unit, turns out I still can't build pressure. Going to replace governor but I think my issue is with the compressor. Will try gaskets after governor. Will keep everybody posted as to what the problem was when I finally find it.

 

[The HUlk]

I am having the same issue with not building pressure past 30psi, I did find air steadily coming from the air dryer purge valve. Listening to others advice, I replaced the dryer with a brand new unit, turns out I still can't build pressure. Going to replace governor but I think my issue is with the compressor. Will try gaskets after governor. Will keep everybody posted as to what the problem was when I finally find it.

I had trouble this summer with building air and had to replace my air dryer and use 400grit sandpaper to clean sticky residue from the air compressor unloader valve to solve the problem. The story and details can be found here...

http://www.steelsoldiers.com/showthread.php?141256-M939-Inconsistent-air-pressure

 

[The HUlk]

Here is another thread that covers solving air pressure building problems. It's a long read but contains some good info...

http://www.steelsoldiers.com/showthread.php?141771-Air-Fails-To-Build-Intermittently-Parking-Brake

 

[WSm925a2]

Here is another thread that covers solving air pressure building problems. It's a long read but contains some good info...

http://www.steelsoldiers.com/showthread.php?141771-Air-Fails-To-Build-Intermittently-Parking-Brake

Great, thanks for the info! So far, mine builds 30 psi weather engine is warm or cold. I replaced the air dryer and it built to 40 psi then built no further... Still reading, ordering parts, and troubleshooting. Thanks again for the threads, great info!