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Old 06-29-2019, 07:12 PM   #1
Mini-Skoolie
 
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Air brake issue.. any ideas are welcome

QUOTE]This has happened twice today. The air brake pressure drops and drops... The Alarm sounds.

The first time, I almost turned back, but then pressure built again... Ok.

This happened a second time today, this time it didn't bounce back and I got myself to a gas station parking place with minimal pressure. My parking break released itself as I arrived... Scary.

I open the hood look around and asked my wife to turn it off then back on... It starts to build pressure again... Any thoughts?

Thanks in advance![/QUOTE]
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Old 06-29-2019, 07:49 PM   #2
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Hopefully you are close to home or a repair shop!
It definitely sounds like an air leak, Trouble is finding it can be an exercise in frustration.
Assuming you didn't run over something just before it happened, I'd start in back underneath checking the rear brake hoses following them forward while your wife sits inside with engine running. Any sounds of air leaking anywhere along the way is likely where your problem lies.
There could also be a compressor issue. An under hood belt check is in order.
There are many ways to check compressor functionality/output in the archives on here, so I won't add redundant information to confuse you.
Good luck in diagnosing this definite safety issue...
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Old 06-29-2019, 08:00 PM   #3
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Did it pass the 4 point air brake safety check during the pre-trip inspection?
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Old 06-29-2019, 08:19 PM   #4
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Quote:
Originally Posted by Sehnsucht View Post
Did it pass the 4 point air brake safety check during the pre-trip inspection?
Exactly

One of the better YouTube videos I found describing the process that's school-bus specific:



I'm new to air brakes myself, but from what I've tought myself, it could a leak, a problem with the governor, user error, or something else. But the pre-trip test should both confirm whether or not your system is working properly, as well as give you an indication of what the problem is if it isn't.


Lots of resources out there describing how air brake systems work.
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Old 06-29-2019, 10:07 PM   #5
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My question is - did you hear air actually leaking, either through the parking brake valve or ... anywhere else? I've had a truck or two develop intermittent air-pressure loss similar to this and in both cases it was the air dryer purge valve sticking open. It would close sometimes, but if/when it stuck open, it would lose all air pressure.
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Old 06-29-2019, 11:28 PM   #6
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The real truth is you do not know anything about air brakes and now they are in emergency failure mode you pull over and the e brake locks!!Tow that bus and spend some money to have the brakes fixed !!!!!!!!!!!!!!!!!!!!!before tragedy happens as I see it you have had 2 warnings....
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Old 06-30-2019, 12:05 AM   #7
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Quote:
Originally Posted by TheHubbardBus View Post
Exactly

One of the better YouTube videos I found describing the process that's school-bus specific:

This looks like a good procedure, the only thing I would comment is I think she was unclear about the static tests the engine is not running but the ignition needs to be in the ON position otherwise these lights and buzzers won't be doing anything. Beyond that this looks like a thorough procedure and confirms before any trip that the vehicle is safe and properly functioning. Of course the one other aspect is this video assumes all air compressors are gear-driven but older buses could instead have a belt-driven air compressor so there is a wear factor there that could come into play during operation that doesn't necessarily manifest in the test stage. Like any belt, this should be visually inspected for slack and wear signs. Otherwise, a gear driven system would have no under-hood mechanicals to inspect. It would therefore be vital to know which type you have in order to know whether there's a belt that could wear, slip, or otherwise create the situation described by the OP.

Therefore, having done these steps each day, the OP can rule out stuff like a wear-related constant leak and must consider what accessorial components can produce a sufficient amount of air leakage that exceeds the compressor's ability to replenish leaking air. This can be something like a sticking purge valve but without knowing what features are on a particular bus its hard to say where to look first. The last time I had a tell-tale audible air leak it was a leveling valve for the truck's air suspension but this is unlikely on most school buses which doesn't even use air ride.

I would suggest as a diagnostic procedure parking in a safe place, chock the wheels, run the air pressure up to the cut-out pressure (120 psi), shut the engine off, then ignition on but engine not running, push the parking valve in, and step from engine compartment to all four wheels listening for air hissing. If you have assistance, have assistant apply brake pedal and repeat last step from engine compartment to all four wheels listening for air escaping. In a bus or other non-combination vehicle, the allowable loss of air pressure during brake application is less than 3 psi per minute so any air leak large enough to deplete the air supply in spite of the air compressor output is not going to be difficult to locate as it will be loud and obvious. Next steps depend on the outcome of this procedure.
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Old 06-30-2019, 02:11 AM   #8
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Quote:
Originally Posted by Rivetboy View Post
The real truth is you do not know anything about air brakes and now they are in emergency failure mode you pull over and the e brake locks!!Tow that bus and spend some money to have the brakes fixed !!!!!!!!!!!!!!!!!!!!!before tragedy happens as I see it you have had 2 warnings....

Not very helpful. Obviously something is wrong - and I'm operating under the assumption it is *NOT* the actual brakes themselves. It's the air system which makes the parking brakes release and service brakes function. I'll agree our OP should learn how the system works and how the air system works (and is supposed to work) but right now he needs to get it home.


Quote:
Originally Posted by Sehnsucht View Post
..... The last time I had a tell-tale audible air leak it was a leveling valve for the truck's air suspension but this is unlikely on most school buses which doesn't even use air ride.

Many buses these days do have air ride suspensions. But for those that don't, correct there will be no leveling valve.


Quote:
Originally Posted by Sehnsucht View Post
I would suggest as a diagnostic procedure parking in a safe place, chock the wheels, run the air pressure up to the cut-out pressure (120 psi), shut the engine off, then ignition on but engine not running, push the parking valve in, and step from engine compartment to all four wheels listening for air hissing. If you have assistance, have assistant apply brake pedal and repeat last step from engine compartment to all four wheels listening for air escaping. In a bus or other non-combination vehicle, the allowable loss of air pressure during brake application is less than 3 psi per minute so any air leak large enough to deplete the air supply in spite of the air compressor output is not going to be difficult to locate as it will be loud and obvious. Next steps depend on the outcome of this procedure.

In another thread I touched on this, but I'll repeat it here. Buses are large heavy vehicles and more than one has pinned a driver checking (or messing with) the brakes. You will want to make *ABSOLUTELY SURE* it will *NOT* roll while you aren't in the driver's seat. A helper is best, but chocking the wheels may have to suffice if you're alone.


In any event, it seems our OP has more than just a 3 PSI per minute leak, and it's not something constant. It seems to be an intermittent thing, if I understood the description correctly, which indicates to me something that (A) only happens intermittently and (B) causes a major air loss. A leveling valve, in my experience, doesn't deplete air that fast, and will leak constantly once it has worn that much. No, I suspect something else, and I stand behind my air dryer suspicion at least until it's checked (or for an older bus, found not to have one).
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Old 06-30-2019, 07:30 AM   #9
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Thank you all for your feedback. It was a bit scary for a while there. I think it might be one of two things.

There is one think I also noticed which I didn't mention before... it's just an ever so slightly difference. When the brakes are charged and there is the air release... It's not very different but it seems it releases slower, instead of "psst!", It's like "pssst!"

It's not twice as long or anything, but just a bit longer.

I route was through New Mexico and Arizona. Incredibly mild on the way there. Winslow hit 48 overnight. In the 80s during the strange cool hit last week.

I parked it for the week at KOA, the temps were mild. AC froze us out if the sun wasn't shining...

Could any of this be altitude related?
If not, the release valve is in question.

2nd. When no pressure is going into the system, the governor has got to be the culprit, right?

Sitting in Gallup NM, and just want to get my family safely home.
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Old 06-30-2019, 09:51 AM   #10
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1. Read this twice - are you referring to the air purge once it builds up pressure? Instead of a quick hiss of air, now it seems to stick a little? That also sounds like an air dryer valve could be sticking.


2. No, not always. There's another thread on the forum addressing no air pressure and changing the governor did not solve his air issue. In his case, it's been established the air compressor is pumping air (indicating a the governor works too) but something in the remainder of the system isn't holding it.


I can't think of many components in the air system that could both (A) cause a major air loss and (B) only do so intermittently.
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