I've got to agree with the others here, check the purge valve and the governor. I had a similar problem with the dryer on my Freightliner. It was about time to replace the dessicant cartridge so I did that and rebuilt the purge valve at the same time. The problem of the valve staying open for an extended period got a bit better but never went away. I then started to notice that if I tapped the brakes a few times and made the air pressure drop enough for the governor to allow the compressor to start building air again it would stop. In my case I had also started having issues with the brakes seeming weak when applied. The guage on the dash read equally for both the primary and secondary sides of the system, and the low air buzzer came on around 70 psi, but the brakes still felt weak. When I installed a different guage in the system I found I was only running around 105 psi system pressure, so I jacked it on up to between 125 and 130 where it should have been all along. Now it reads nearly 150 on the dash guage, the low air still goes off around 70, and the purge valve does it's thing without staying open too long like it had been doing.
That said I know on mine the purge valve senses system pressure and cycles based on what the governor is doing. I can't say exactly how it goes about the cycle but, based on my experience, it's cycle time open must have something to do with a pressure differential in the system. In my case, with system pressure too low (at a certain pressure point anyways), it stayed open way to long and would not shut on it's own. However if I made the pressure drop a bit more so the governor kicked in it would close almost immediately when the compressor started pumping and system pressure started to rise.
I know this whole deal about drove me crazy trying to figure out since I was trusting the air guages on the dash to begin with. Once I used a third, known, guage to look at things with a different eye, everything fell into place and I haven't had any more problems out of my braking system as far as the purge valve goes, and the stopping power is now better than it's been since I bought the truck since it's actually running a proper pressure now vs being nearly 20 psi low.
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Today's Featured Article - Show Coverage: Journey to Ankeny - by Cindy Ladage. We left Illinois on the first day of July and headed north and west for Ankeny, Iowa. Minus two kids, we traveled light with only the youngest in tow. As long as a pool was at the end of our destination she was easy to please unlike the other two who have a multitude of requirements to travel with mom and dad. Amana Colonies served as a respite where we ate a family style lunch that sustained us with more food than could reasonably fit into our ample physiques. The show at Ankeny
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