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Old 03-10-2019, 05:14 PM   #1
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Loss of air compression

I just recently bought a 1999 GMC/Bluebird bus with the cat 3126. I went to start it and noticed I have no air compression. I'm new to buses but am a decent home mechanic looking for advice. I'm going to start looking for leaks but is there anything else to look for?
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Old 03-10-2019, 06:06 PM   #2
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The governor (that's the valve generally bolted to the side of the compressor, which cuts the compressor in and out, as needed), can stick, as well as the dump valve on the dryer if you have one.
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Old 03-10-2019, 06:34 PM   #3
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you have not given much info where are you? what is outside temp how long did you run it? If it is extremely cold Gov could be stuck,air drier frozen water in an air line frozen if wasn't run for very long could be didn't build enough air to show on gauge gov is set to come on at a certain low pressure did it get low enough to trigger gov? are both brake valves pulled?
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Old 03-10-2019, 07:06 PM   #4
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was it that after you started the engine the air gauges failed to come up from 0?

sometimes people drain their air tanks overnight... underneath the bus are usually a valve on the bottom of each air tank, make sure they are all closed so the pressure can build up...

if the air pressure is just on zero before you start it, go ahead, start it and see if it builds up... it is quite common for the air to leak down overnight..

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Old 03-10-2019, 08:28 PM   #5
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Perfect timing for this thread as I am having a similar problem.

Started my bus today and the air pressure built up to 90+ no problem. I decided before I drove I wanted to bleed off my brakes as it has been sitting for a moment in colder temps. The air pressure was fine until I pressed the brake pedal and the pressure continued to drop so I went back to the valves as I figured one was not shut correctly. The valves were shut and the pressure will not build past 30 psi. Also the tank had oil and water when I drained it, ugh. Any ideas?
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Old 03-10-2019, 10:27 PM   #6
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change the dryer filter.

ime there are 2 reasons for oil in the tank. air drier filter overloaded/neglected or the check valve leaving the compressor has gone bad.

an unserviced air dryer filter would be most likely.

bendix has some great troubleshooting diagrams online to help you sort out air systems
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