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Turns out it was something to do with the unloader valves in the air compressor. We bought a rebuild kit and threw it in and now everything works as it should. I think one of the little springs in there was busted. This was back in May. I just forgot to update this thread until now. Also, this...

We replaced the governor and air dryer purge valve since both were cheap so why not. The bad noise (from my last post) went away. The truck built around 100-120psi and we heard the automatic air release. All seemed well. But then we went to drive the truck and noticed the air pressure was now...

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So the truck went from building no air pressure to plenty of it. My friend messed with the governor I think. But now the truck makes an awful noise once fully up to pressure. It sounds like its coming from this thing:

Thanks for the replies. What/where is the 'auto purge'? Isn't the unloader the same as the governor? Google is seeming to suggest that to me, and I don't see anything on the other side of the compressor.

M985 HEMTT won't build any air pressure. Starts and runs fine, but will not drive as the brakes (and apparently throttle) are air actuated. Air pressure gauge reads zero. Thinking compressor but can't find any spare replacements. But maybe it's not even the problem. Any ideas?