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Old 02-19-2012, 09:02 AM   #1
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Join Date: Feb 2011
Location: Port Huron,Mi
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air pressure problem

Still having air pressure problems on 2004 Mandalay, freightliner chassis with a 350 cat.
Can not build air past 80lbs. Replaced governor and changed dryer filter and checked for leaks on everything by sound and soap. Finding none. To even build air to 80lbs. have to run at about 1300 rpm's. Lightly tapped on all solenoids etc. makes no difference. Only seems to build air in one tank very little in the other. Air pressure bled off about 6 or 7lbs. overnight. anyone have any other ideas for me to try before calling in mobile repair?

Rick & Alyce
04 40B Mandalay 350 Cat
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Old 02-19-2012, 10:30 AM   #2
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I drive a school bus and many times I hear drivers call in to dispatch that they can't build up air pressure. There is a mechanic on duty at dispatch and the answer is, turn off engine, bleed the air system down and restart. You don't say what temperature it is but I usually hear this in cold weather. I'm wondering if water in the system has turned to ice blocking the pressure. They say to continue doing this until freed. Sometimes 20 minutes or so. It did happen to me once and after bleeding it down and restarting, it worked enough to release the barking brake. I traveled slowly to warm up the engine and finally air pressure came up (1 mile).

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Old 02-19-2012, 02:21 PM   #3
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You could try putting some air brake antifreeze in the line off your air dryer and working that through the system. It sounds like one of your valves or airlines may have ice in them. If that doesn't fix it it may be the unloader valves in your compressor for which I would take it to a good shop.
Dennis & Marcie & Hook The Jack Russell, 2001 HR Imperial 38wds 350 Cummins, 07 Chevy Trailblazer/Blue Ox/Ready Brake, 04 Dodge 3500 Cummins,
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Old 02-20-2012, 05:54 AM   #4
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Thanks for responding but I'm in Florida, so it is not ice. Could be a stuck valve somewhere. Hopefully not the compressor.
Thanks, Rick
Rick & Alyce
04 40B Mandalay 350 Cat
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Old 02-20-2012, 09:38 AM   #5
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Moisture in the system reduces air capacity. It also causes sludge buildup. A worn belt will slip as the pressure against it increases reducing it's effectivness. Tightening it may help. A leak anywhere in the system will reduce pressure as well. I believe there could be a problem with lubrication of moving parts reducing the ability to compress. As the air pressure builds it requires more force on things to make air. All this is from a long time user but one who always calls a mechanic that knows beyond spectulation, something that I am good at.
You may want to check the gauges for accuracy as well.
Larry B, Luckiest Dreamer
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