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Old 07-03-2021, 02:07 PM   #1
Mini-Skoolie
 
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Help - Slightly different brake pressure problem

So like MeyerMobile's problem this week, I am in an air pressure situation. With slightly different symptoms. But with similar disturbing cycling of the air pressure between 90 and 120 psi without any braking activity to explain the air losses.



Because I don't know much and thus don't contribute much to this site, I'll offer a $50 bounty to the first correct diagnoses (paypal account of your choice). You can donate it to skoolie.net if you wish.


Anyway, I'm currently in San Antonio on the way to Colorado.
Bus is 2006-07 IC CE 300 wheelchair bus (8 window). DT466e, Allison 2000... Appears to be mostly bendix air components with 2 tanks, anti-lock brakes, air door, air captains seat. Two dash gauges, Tank 1 and Tank 2. Wheelchair lift has been removed.



Symptoms:
1) Low pressure alarm, 90psi kick in and 125 psi pop off all seem operationally correct as indicated on both gauges.


2) According to the gauges, Tank 1 appears to lead Tank 2 by a few psi (< _5) when discharging, and lag a few psi while charging. This might be irrelevant.



3) With the parking brake applied I believe the system is acting normally. it holds pressure and I can find no significant leaks.


4) With the parking brake released the pressures on both tanks cycle every 45 seconds or so between 90 and 120 psi. So there is a pop off every 45 seconds at 125 psi.. And then 5-10 second rapid fall back to 90 psi. This is the problem..


5) I have sprayed soapy water everywhere and cannot find a leak. I cannot hear a leak with the engine running. If I turn off the engine with parking brake released, then a safety valve on the firewall dumps all the air preventing me from hearing anything.



Thank you in advance for your help.
Phillip

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Old 07-03-2021, 02:50 PM   #2
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You have stated a clue to the problem "With the parking brake applied I believe the system is acting normally. it holds pressure and I can find no significant leaks" When you release the parking brake you actually apply pressure to your air chambers. The air chambers that have the parking brake have two chambers, one that works against the powerful springs that actually apply the parking brake, and the other one that applies your brakes to stop. When you apply the parking brake, you release the air pressure that holds back the springs that apply the parking brake.

Based on your statements, I would check the following:


1) Both air chambers that apply the parking brake (normally the rear ones).


2) The air lines that supply the chambers on the parking brake side.


3) The release valve on your dash (big yellow one).


The fact that you can build pressure is because your parking brake is applied as you fill the air system. In this condition the probable place of the leak is not pressurized.



Do NOT drive your bus until this is fixed. When the failure gets bad enough, you will experience brake lockup wherever you are.


CAUTION!!!


If you remove any air chamber with parking brakes properly "cage" the chamber first using the supplied special threaded "bolt" mounted to it. If you don't do this first and loosen the clamps that hold the air chamber in place, the chamber will violently come apart with enough force to cause major injury or worse.


If I am right, I don't want any money for it.
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Old 07-03-2021, 03:11 PM   #3
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Thank you Flattracker, your suggestions make perfect sense.

For now, i'm kinda ignore the yellew parking brake pushbutton on the dash because I can't hear any leaks from it and soap spray in the firewall behind it reveals no leaks.

And I've sprayed soap thoroughly for and aft on all the feeder lines....

So I'll focus on the double stack chambers on the rear axle.
They are brand new.. i will investigate further. Perhaps somebody didn't tighten something down..

Regards,
Phillip
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Old 07-03-2021, 03:31 PM   #4
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Quote:
Originally Posted by MonsterMoto View Post
Thank you Flattracker, your suggestions make perfect sense.

For now, i'm kinda ignore the yellew parking brake pushbutton on the dash because I can't hear any leaks from it and soap spray in the firewall behind it reveals no leaks.

And I've sprayed soap thoroughly for and aft on all the feeder lines....

So I'll focus on the double stack chambers on the rear axle.
They are brand new.. i will investigate further. Perhaps somebody didn't tighten something down..

Regards,
Phillip

Keep in mind that if the parking brake was applied when you were spraying soap on the lines, that they were NOT pressurized. PUT CHOCKS AT YOUR WHEELS so your bus won't roll, release the parking brake and then test/check your lines and chambers. This sounds like a large leak. It won't be hard to find.
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Old 07-03-2021, 03:54 PM   #5
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Yes that's the way I've been approaching it. Big wheel chocks while in air leak mode...
It's very odd that I'm having trouble isolating such a big leak...
Thank you for your help.
Phillip
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Old 07-03-2021, 04:01 PM   #6
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We know that the soap spray is good for small or moderate leaks. Maybe it's such a leak that blows the soap water away without a bubble. I'll add more soap to the mixture and maybe plug the safety dump valve so that I can explore without engine running....

Thanks again for your time and suggestions on this holiday weekend.
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Old 07-03-2021, 04:09 PM   #7
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Both tanks dropping and rising almost together does that tell us anything interesting? Or is that still indicative of a demand side leak as we have discussed?
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Old 07-03-2021, 04:26 PM   #8
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Everything you have stated so far indicates that when you are building air pressure, your parking brake is set/not pressurized. Your problem is a leak in your parking brake system best I can tell. temporarily closing the dump valve on your firewall seems like a good idea. I don't know why you would even want such a device. A good tight air system will keep most of the pressure all night. Definitely you want to periodically dump your air so you won't build up water in the air tanks.
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Old 07-06-2021, 09:16 AM   #9
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Look at the relay valve for the park brake in the rear. It will have access to both primary and secondary air pressure in case of failure of either of those systems.

As said, CHOCK THE WHEELS

I've got a large dent in the side of the shop truck where a bus rolled out of the shop while doing similar diagnostics. Luckily the truck was there, or I on the creeper would have been the next stopping obstacle.

Next, release the parking brake and walk towards the back of the bus. If you're leaking that much air, it should be audible, rendering a soapy water solution unnecessary. If you have to leave the engine running, then do so. Just make sure you're on flat ground with the wheel chocked.
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Old 04-25-2023, 11:13 PM   #10
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I'm not sure I ever thanked you guys for the help on this.
In the end, I got enough confidence with the leak and it corrected itself enough over time to allow a wonderful family Summer vacation to Colorado and up and down several steep passes.
But on the way home (to central Texas) the bus was T-Boned at an intersection in Lubbock.
And soon totaled by the insurance company.
So I'm back working on my big RE bus.
Thanks again to those that helped us.
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Old 04-26-2023, 02:08 PM   #11
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T-boned

What of the bus occupants condition after the t-bone?

William
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Old 04-27-2023, 12:14 PM   #12
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Alarmed (they looked up from their phones) but nobody was injured.
We replaced two tires and continued the journey to San Antonio.
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