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Old 09-20-2021, 08:46 AM   #1
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Air tank breake pressure question

Hi everyone!
2005 Thomas Saf-T-liner, Cat c7/allison.
I am still working on the bus so don't drive it at all. But I do start it most every weekend and let it run for a few minutes to fill up the air tanks.


This weekend, it was 95 degrees, when i started the bus it barely got to 50psi and the alarm finally turned off. I mean this seemed like 10 minutes. I was running the air inside at the time on low, as it was really hot.
The brake air pressure never got much above 50. Every time I hit the brake, the alarm went off, and the pressure dropped to 45-48 ish, then slowly bade it back to 50, not much more.


Any ideas what might cause this? I looked under the bus for anything obvious, like air draining, etc, and saw/heard nothing.


I have never maintained the brakes, so I don't know if i should be draining the air tanks. There are 4 valves on the side of the bus for doing that i guess.


The bus was delivered [driven] to me from Az to IL about a year ago, it it was PM'd up to that point.


Just looking for some insight here, and thanks!


Peter

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Old 09-20-2021, 08:54 AM   #2
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You've got a leak in the system somewhere for sure. Let it get up to as high a PSI as it's going to go, then shut off the bus and go underneath. You should hear hissing coming from somewhere.
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Old 09-20-2021, 09:10 AM   #3
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Will do. Funny thing is, I drove the bus to its new parking spot 2 weeks ago, and it was fine. Any thoughts on the valves on the side of the bus, I assume those are for draining the tanks? Should I be doing anything with those ?
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Old 09-20-2021, 10:10 AM   #4
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I had similar symptoms that got progressively worse over two or three startups. At first I couldn't find the leak, but as it got worse I started to be able to hear it and finally found it. In my case it was the exhaust port on the air governor. Replaced the governor and all's well again. Since then I've learned it's reasonably common for governors to go bad. Maybe start your search there? If listening doesn't work, spray everything down with sudsy water.
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Old 09-20-2021, 10:16 AM   #5
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Thanks Tejon, I'll start by draining the tanks. I have 3 valves, and a accessory valve, which never has air. Then me and soapy water will look for the governor. Im assuming its near the brakes somewhere.
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Old 09-20-2021, 10:18 AM   #6
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If you have air suspension also check the air bags and leveling valves. The bags can just rot while sitting and then leak.
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Old 09-20-2021, 10:21 AM   #7
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Thats a good point. Soapy water again I assume. I was under the bus yesterday , when the pressure was low, 50 psi, and didnt see or hear anything. As I said, I never drained the tanks for over a year now since I got the bus.

I found this link which describes my air brake system: https://www.skoolie.net/forums/f33/w...ves-29185.html
"an occasional discharge of air that lasts half-a-second or so, it's likely an air dryer doing exactly what it's supposed to be doing"

Now that I remember, yesterday , I didnt get that occasional discharge of air, probably cause I wasnt building up enough air pressure in the tanks. Two weeks ago, I did.
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Old 09-20-2021, 10:33 AM   #8
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The governor will be near the compressor.
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Old 09-20-2021, 10:35 AM   #9
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The air governor should be right on the compressor, so somewhere in your engine bay. Not saying that's definitely the problem, but it ought to be one of the easiest things to access and check, probably without even crawling underneath.

I'm no expert, but if the air dryer is never purging, that makes it sound like the problem is somewhere in-between the tanks and the compressor? In other words, not downstream of the tanks.

Edit: I was too slow with the compressor location
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Old 09-20-2021, 11:02 AM   #10
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Ok, thanks everyone, I'll scout this issue out when I get back out to the bus today or tomorrow, and will post what I found.
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Old 09-20-2021, 11:34 AM   #11
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Again, it the air dryer, always purging, till yesterday, when the psi only made it to 50. Im guessing it wont purge at a lower psi, just a guess.
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Old 09-20-2021, 12:40 PM   #12
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The location of the governor will vary. On my 1986 international dump truck the governor is located on the compressor, but in the "new Crown" it is mounted to a crossmember on the frame several feet from the compressor, which is mounted to the "down" side of the engine (a Cummins big cam 400). I recommend getting a brand name governor such as a Bendix, also. I had problems with one from NAPA. (it was from china)
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Old 09-20-2021, 12:46 PM   #13
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One more thing: I ended up replacing all six dump valves in the "new Crown" as all were leaking some. This was found during the engine swap. I have also replaced the air dryer, and one of the check valves in the air ride system. Air systems can be a pain with an older bus. I have a friend who works for Cal Fire maintaining their fire trucks and he says they are happy with 65 psi left in the system after overnight.
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Old 09-20-2021, 12:49 PM   #14
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The dryer only purges at compressor cut out. If you're not getting enough air pressure(around 120psi) in the system to cause the governor to initiate compressor cut out, you'll never have a dryer purge.

Dryer purge should only last a handful of seconds. If it's continuously flowing until compressor cuts back in, your check valve on the dryer outlet has failed. The purge valve should only purge the air/moisture contents of the dryer. The wet tank pressure should remain stable during the purge sequence.

If the dryer is continuously flowing when trying to build air pressure, your purge valve is stuck open, or has failed open. Someone on here recently had that problem and "solved" it by whacking the valve with a hammer. I don't recommend doing that, as purge valve rebuilds are cheap and should be done every few years minimum.

If you can't build pressure, it's most likely due to a leak, so look for it.
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Old 09-20-2021, 12:56 PM   #15
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I agree with Boyah about air dryers. When they purge they make a fart type noise. the original air dryer in the "new Crown" had a leak where it had a low his noise that continued after engine shutown. It could have been rebuilt but it was a push between replace with new and rebuild kit. They will "fart" once sometimes after shutdown, but that is different than slow hiss of air leaking out.
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Old 09-20-2021, 01:17 PM   #16
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Farts like an air dryer purge should be immediately followed by a call to your proctologist and a trip to Kmart for some new pants
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Old 09-20-2021, 02:02 PM   #17
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Quote:
Originally Posted by Tejon7 View Post
Farts like an air dryer purge should be immediately followed by a call to your proctologist and a trip to Kmart for some new pants
Especially from the wet tank...

EDIT: I know that made no sense from either a biological sense or an air system sense...but go with it.
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Old 09-20-2021, 02:30 PM   #18
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Originally Posted by Peterjk View Post
Will do. Funny thing is, I drove the bus to its new parking spot 2 weeks ago, and it was fine. Any thoughts on the valves on the side of the bus, I assume those are for draining the tanks? Should I be doing anything with those ?
Hi Peter
You should be draining those tanks every time the bus has been running. compressed air is hot and as it cools going to the tanks it is creating condensation. You are draining them to remove the water that will eventually rust your tanks out but you also don't want that moisture anywhere in you air system(think about freezing climates)
Cheers
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Old 09-20-2021, 02:32 PM   #19
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When under the bus looking for leaks please chock the wheels!
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Old 09-20-2021, 03:08 PM   #20
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Air tanks should be drained daily. They will get condensed air in them just sitting.
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