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Old 02-01-2021, 11:16 AM   #1
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Air Dryer is Purging Air through Unloader Port (Line to Governor)

My air brake system on my Bluebird TC2000 is not pressurizing. I've checked all the lines and fittings and haven't been able to find any leaks. What I have found is that the governor is purging air constantly through its exhaust port, and this seems to be happening because there is air coming FROM the air dryer and entering the governor at the unloader port (the port in the middle of the governor).

From everything I've seen, my governor is plumbed correctly. I've got a line going to the air compressor unloader valve, a line going to the air dryer, and a line going to the wet tank. I believe there should only be air coming into the governor from the wet tank, but I am getting air coming off the dryer.

I unhooked everything from the dryer except the supply line, and the air dryer is just purging all the air out through the port that was connected to the governor. I believe air should be coming IN through this port, not out.

I've disassembled the air dryer and inspected everything and can't identify anything obviously wrong that would cause the air to come out through this port. I replaced the dryer filter recently too.

I've Googled and Googled and haven't been able to find a single other instance of air coming OUT of the dryer unloader port. It's the port labelled "control port" in this diagram:
https://www.anythingtruck.com/commer...ss-Section.png

I have a Bendix AD9 dryer, and a bendix governor.

Has anyone seen anything like this before, or have any tips on tracking down the culprit? Do I just need to replace the whole dryer?

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Old 02-01-2021, 11:49 AM   #2
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Great troubleshooting. You shouldn't have any air coming from the control port on the dryer.

If you do, you likely have a faulty purge valve, or O-ring on said purge valve which is causing air to leak between the supply and control port on the dryer.

Either replace the valve or replace the drier assembly. We typically replace the assembly with a reman, but you can rebuild these as long as the housing and assembly is in okay shape. If it's corroded or cracked, I'd replace it with a reman.
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Old 02-01-2021, 12:59 PM   #3
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Thanks for the tips! I'll try inspecting the purge valve again, and replacing if necessary.

EDIT: Purge valve looks to be fine from all visual inspection. All o-rings are present and intact and all the parts seem to be moving as they should

I just can't figure how air is leaking from the supply port to the control port.

It does look like the little rubber seal (check valve) on top of my air dryer filter is missing. This could allow air to flow backward into the filter... I wonder if that could be what's causing the backflow through the control port?
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Old 02-01-2021, 02:06 PM   #4
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You disassembled the purge valve and checked for piston movement, o-ring location, and that the spring was located correctly?

What's the housing look like where the purge valve installs?

Truthfully, it's not that complicated. If you're leaking air between the control port and the supply port, the purge valve is at fault or the housing is damaged.
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Old 02-01-2021, 02:07 PM   #5
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At no point during operation is the control port and supply port connected, So even with the spring missing, you'll still have a separation between the two by an o-ring.
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Old 02-01-2021, 02:17 PM   #6
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Okay, understood. So it couldn't be related to the missing check valve on top of the dryer cartridge?

Yep, I disassembled the whole purge valve. I already put it back together, but I can do it again and get pictures of it later. Looked just like this picture.


All three of those O-rings were present and in the right spot, looking like they're in great condition. The piston presses down from on top and springs right back up. The O-ring near the top of the piston ("quad ring" in this diagram) is in place and seems fine. I have the piston tightened up as tight as it will go against the nut on the bottom of the valve. Does that need to be adjusted at all? Right now the top of the piston is flush with the top of the valve housing.

The housing around the where the purge valve installs has some old mud caked around it, but isn't cracked as far as I can tell, and I can't see any air escaping when I spray it with soapy water. I may clean it up and get a closer look.

I've got a new purge valve on order to arrive Wednesday, in case I can't crack the code by then.

Thanks again for the help.
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Old 02-01-2021, 02:34 PM   #7
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What check valve are you missing? The only check valve in the system is by the discharge port, which prevents air from the wet tank/line flowing back into the dryer.

That won't cause you to not build pressure though.

If you have air flowing out of the control port of the drier, then the only culprits are the purge valve assembly or the housing itself. Here is a cutaway of an ad9 drier.

The 2 o-rings that separate the supply to the control is on your diagram as the small o-ring and quad o-ring.

Take a picture of the top of your purge valve assembly showing your quad o-ring, maybe it came off the piston and is leaking.
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Old 02-01-2021, 02:35 PM   #8
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Can you disassemble your purge valve and maybe get a better view of the piston and quad o-ring? It's hard to see as installed.
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Old 02-01-2021, 02:46 PM   #9
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https://youtu.be/sBwKvY8e-Pg?t=225

This is a video of an ad9 rebuild. At 3:00 he starts to rebuild the purge valve. At 3:45 he shows the install of the quad ring on the piston. The bolt that holds the purge valve to the piston isn't adjustable, just get it tight.
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Old 02-01-2021, 02:46 PM   #10
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I'm referring to the little rubber cap that sits right on top of the dryer cartridge:


I saw it labelled as a check valve on another diagram I can't find now. t seems like air is supposed to flow up through it and not be able to get back through. I wondered if it was backing up through the filter somehow because that's missing.

I'll get some pictures of the o ring and piston here soon when I can get back outside.
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Old 02-01-2021, 02:49 PM   #11
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That cap is supposed to be removed. It's sole purpose is to keep the dryer sealed and desiccant fresh until it's installed. You're supposed to remove that before installing the cartridge in the dryer.

During purge, dry air inside the housing is supposed to flow back through that hole, through the desiccant, carrying the moisture in the desiccant out through the exhaust port.


EDIT- Just looked at a cartridge on the shelf, and that is a little check valve on top of there. Not sure why it's there but it isn't important for operation and wouldn't cause the issues you're having.
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Old 02-01-2021, 03:37 PM   #12
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Pictures of the unit and housing.

It seems like the quad ring is a little bit worn. But I don't see any obvious reasons for such a huge leak.
Attached Thumbnails
20210201_162953.jpg   20210201_163010.jpg   20210201_163034.jpg   20210201_163206.jpg  
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Old 02-01-2021, 03:49 PM   #13
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Your quad ring is a lot of bit worn. There is supposed to be a lip on the top and bottom which isn't there. Air is compressible, so it doesn't take much of leak size to lose a lot of it.
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Old 02-01-2021, 03:51 PM   #14
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Aha. That must be the culprit! Any idea where I can buy jus a quad ring?
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Old 02-01-2021, 04:02 PM   #15
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Quote:
Originally Posted by Danielaurence View Post
Aha. That must be the culprit! Any idea where I can buy jus a quad ring?
No not just the individual ring. They offer maintenance kits for the entire purge valve, which is what I would do because all of your o-rings look a little square, when they should be round. The video I linked showed the guy installing a maintenance kit. The problem is, the last time I checked, bendix maintenance kits cost just as much as a reman'd purge valve assembly.

https://www.napaonline.com/en/p/MBI5005893K

That's a link for the maintenance kit.
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Old 02-02-2021, 09:13 AM   #16
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Thanks for the help. I found a maintenance kit at a local auto store and will be attempting the install today. Wish me luck!
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Old 02-02-2021, 10:27 PM   #17
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Originally Posted by Danielaurence View Post
Thanks for the help. I found a maintenance kit at a local auto store and will be attempting the install today. Wish me luck!
You have to follow up, people are curious, it's the law 8-}
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Old 02-04-2021, 10:16 AM   #18
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That was it! I swapped out the O-rings, and now she's building up pressure like a champ.

Thanks again for the help.
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Old 02-04-2021, 11:06 AM   #19
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Awesome, good job
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