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Old 05-22-2020, 06:39 AM   #1
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Air Brake Pressure Won't Build Up

Quick backstory:
I bought my bus, a 1995 Bluebird TC2000, about a year ago, and drove it home from Georgia, about a five hour drive. During that drive I remember the brake air pressure not getting much higher than 60.

After parking it, because of bad batteries and later some engine startup issues (maybe some laziness too), I didn't get the thing started until a few days ago. So it has been sitting for a while.

When I fire it up now, the air pressure won't build up any higher than about 30-40. I figure I must have a leak in the air system. Probably always have. When I first bought the bus they told me it had been running for a couple hours to "warm up". I assumed that was normal, but I was so naive. It probably had been running that long just to get the air pressure up to a drivable level.

Anyway, when I turn the engine off, I can hear a pretty loud woosh of air escaping from the Bendix D-2 air compressor Governor, as seen in the picture below. The air is escaping through the exhaust port of the governor. Here's a manual I found on the part. It seems like the exhaust port shouldn't be venting unless the air pressure is over 120 or so.
http://www.plazafleetparts.com/uploa...aintenance.pdf

I know these parts can fail, but the one I have looks quite new compared to other parts on the bus. Everything else I can see on the air tank system seems to be rust-free and in order, and I wasn't able to locate any other leaks when I used soapy water to spray things.

I guess what I'm mainly after here is troubleshooting advice. Should I start by replacing the governor, or is it possible that there's an issue elsewhere in the system that is throwing a false high pressure positive and causing the governor to dump?

Any advice or similar experiences would be much appreciated, thanks!
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Old 05-22-2020, 07:15 AM   #2
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I would start with the governor they fail often and are cheap.
I carried a spare in the semi.
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Old 05-22-2020, 08:07 AM   #3
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what's the line on the top go to? Take another picture from the top and bottom of the governor showing which lines and ports are used.

It looks like that line is in one of the unloader ports. If that's the line from the wet tank, then they have the governor plumbed wrong. If that's the case, you'll never build pressure because air will be constantly leaving the exhaust port of the governor.
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Old 05-22-2020, 08:27 AM   #4
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Thanks for the replies!
I'll try to get a better picture. The governor is in a really awkward and hard to reach spot under the bus, so I wasn't able to get very good pictures. Here's one other I got from a slightly different angle. Does the line with the yellow ring around it need to be moved up to the reservoir port? It looks like it's attached to the unloader port.

I'll see if I can get a better picture today.
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Old 05-22-2020, 10:02 AM   #5
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Quote:
Originally Posted by somewhereinusa View Post
I would start with the governor they fail often and are cheap.
I carried a spare in the semi.
This. Start at the most obvious point of contention and work from there. I'm not necessarily an advocate of keep changing parts as a method of diagnosis but these are inexpensive and prone to failure.
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Old 05-22-2020, 10:37 AM   #6
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Quote:
Originally Posted by Danielaurence View Post
The governor is in a really awkward and hard to reach spot under the bus, so I wasn't able to get very good pictures. Here's one other I got from a slightly different angle. Does the line with the yellow ring around it need to be moved up to the reservoir port? It looks like it's attached to the unloader port.

First thing is I donít know enough and havenít had this experience to give a definitive answer. In reading the operation section of the Bendix pdf I would think that there is not enough pressure from the reservoir or there is a malfunction of the governor that is not allowing the valve to close the exhaust port. I have read from other sources that you could lightly tap the governor to free up a stuck valve.
The line you asked about appears to be in the right location. Here is a pic of mine which should be in the same location. Iíd almost relocate that thing if I went to the trouble to replace it.

https://www.skoolie.net/forums/attac...1&d=1590161628


Your valve does seem to be newish but could have failed again or there is another issue that the seller did not want/could not address. You may have enough slack in the air lines to pull the governor down when replacing. Or, you may be able to remove the plate in front of the governor, it only has four bolts.
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Old 05-22-2020, 11:13 AM   #7
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Quote:
Originally Posted by Danielaurence View Post
Thanks for the replies!
I'll try to get a better picture. The governor is in a really awkward and hard to reach spot under the bus, so I wasn't able to get very good pictures. Here's one other I got from a slightly different angle. Does the line with the yellow ring around it need to be moved up to the reservoir port? It looks like it's attached to the unloader port.

I'll see if I can get a better picture today.
Yes it does. That's definitely in the unloader port, so make sure that line either goes to the compressor or the purge connection on the air drier. Are there any other lines on it and where are they at/where do they go?

That governor could be bad, but seeing how new it looks, and how fast you're loosing air, I doubt a busted o-ring would be the culprit. I'd guess someone put a line on in the wrong spot.
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Old 05-24-2020, 04:08 PM   #8
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I just traced the lines from my governor and they are exactly the same as what Bamabus posted in his picture. The one with the yellow ring is going to what I think is the air compressor on the side of the engine. The bottom line next to that is going to the back of the bus, and the top line is going to what I think is the reservoir (a largish metal tank near the driver side front wheel.)

So from what I can tell, the plumbing seems to be in order. My next step is to just try replacing it.
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Old 05-24-2020, 04:29 PM   #9
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Hook the outlet line from the compressor to a gauge, fire up the engine and see what pressure you have--might as well start at the beginning. then just keep going on down the line checking each device as you go.
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Old 05-28-2020, 05:08 PM   #10
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I haven't checked the air pressure.from the compressor yet because I don't have a gauge, but I did replace the governor... Same issue, so that rules out a fault in the governor. I cranked the engine with all the lines disconnected. My lines are wired exactly like in Bamabus's picture. The line connected to the reservoir has a large amount of air pressure coming from it while the engine is running. The line going toward the back of the bus also has air coming out, but not as much pressure. The line going to the air compressor doesn't have any air coming out. This seems a bit backwards from what I had expected. I thought air would be coming from the compressor line, and not from the others. I must be misunderstanding something about how the system is wired, so I'll do some more research, but does this seem to you guys like the expected behavior of these lines? If there's air coming from the line running to the back of the bus, should that be attached to one of the reservoir ports?
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Old 05-28-2020, 05:26 PM   #11
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Governor is certainly possible, but could also be looking at a bad compressor. I had the same symptoms with a failing compressor on an International ProStar.
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Old 05-29-2020, 10:42 AM   #12
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Quote:
Originally Posted by Danielaurence View Post
I haven't checked the air pressure.from the compressor yet because I don't have a gauge, but I did replace the governor... Same issue, so that rules out a fault in the governor. I cranked the engine with all the lines disconnected. My lines are wired exactly like in Bamabus's picture. The line connected to the reservoir has a large amount of air pressure coming from it while the engine is running. The line going toward the back of the bus also has air coming out, but not as much pressure. The line going to the air compressor doesn't have any air coming out. This seems a bit backwards from what I had expected. I thought air would be coming from the compressor line, and not from the others. I must be misunderstanding something about how the system is wired, so I'll do some more research, but does this seem to you guys like the expected behavior of these lines? If there's air coming from the line running to the back of the bus, should that be attached to one of the reservoir ports?
Where is the line to the back of the bus connected to on the governor? Reservoir port or unloader port?

If it's to the unloader port, and it connects to the air dryer in the back, you likely have a fault in the purge circuit of the air dryer. The compressor and air dryer hoses shouldn't have air coming from them. They recieve air from the governor, not supply it, so air coming from them shows a fault in that system.
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