Air brake issue.. any ideas are welcome

[meyermobile]

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]

[peteg59]

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...

[Sehnsucht]

Did it pass the 4 point air brake safety check during the pre-trip inspection?

[TheHubbardBus]

Originally Posted by Sehnsucht

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:

https://www.youtube.com/watch?v=spBm85WzhfY

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.

[Brad_SwiftFur]

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.

[Rivetboy]

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....

[Sehnsucht]

Originally Posted by TheHubbardBus

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.

[Brad_SwiftFur]

Originally Posted by Rivetboy

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.

Originally Posted by Sehnsucht

..... 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.

Originally Posted by Sehnsucht

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).

[meyermobile]

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.

[Brad_SwiftFur]

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.

[Sehnsucht]

Originally Posted by Brad_SwiftFur

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.

Agreed. And may I add that an intermittently manifesting problem will almost always end up tracing back to some manner of foreign contamination which is interfering with otherwise functional components. The additional info provided by the OP makes me suspect the bus has an auto bleeder but debris has collected in the air reservoir and when it purges it prevents the valve from closing fully. I wouldn't think this alone could cause a catastrophic air loss but if the condition exists in one part of the air brake system then there is likely foreign matter throughout the system.

[Brad_SwiftFur]

Originally Posted by Sehnsucht

Agreed. And may I add that an intermittently manifesting problem will almost always end up tracing back to some manner of foreign contamination which is interfering with otherwise functional components. The additional info provided by the OP makes me suspect the bus has an auto bleeder but debris has collected in the air reservoir and when it purges it prevents the valve from closing fully. I wouldn't think this alone could cause a catastrophic air loss but if the condition exists in one part of the air brake system then there is likely foreign matter throughout the system.

Having driven air brake equipped vehicles for well over 20 years (and worked on a few over the years as well), this has been my experience. The air dryer is the first thing in the system to get air from the compressor, before the air tanks, and as such it's supposed to remove the moisture and oil from the air before it enters the "wet" tank (it doesn't get *All* the moisture, especially once they age a bit). Many have filters/cartridges that are supposed to be replaced periodically (typically once a year in a daily-driver) but frequently it's neglected. Eventually the moisture laden oil makes its way past the filter/cartridge and gums up the auto-valve.

[Rivetboy]

Originally Posted by Brad_SwiftFur

Not very helpful. Obviously something is wrong -

Bad brakes are no joke. .To operate a vehicle with known bad brakes is a criminal. Sorry Brad I guess your Mom doesn't have pins in her arm and her car totaled by someone who admitted in court that he knew his brakes were bad. Imagine for a minute a 30K lbs vehichle with bad brakes t boning you. No way should a vehicle with known defective brakes be in operation. Not to mention the liability issues.

[Brad_SwiftFur]

Originally Posted by Rivetboy

Bad brakes are no joke. .To operate a vehicle with known bad brakes is a criminal. Sorry Brad I guess your Mom doesn't have pins in her arm and her car totaled by someone who admitted in court that he knew his brakes were bad. Imagine for a minute a 30K lbs vehichle with bad brakes t boning you. No way should a vehicle with known defective brakes be in operation. Not to mention the liability issues.

I'm not disagreeing that defective brakes are Bad News and need fixed. As a professional truck driver, I've had a few over the years that I've put out of service because By Gosh if nothing else, I want to be able to stop safely - and for that matter, I want everyone else to be able to stop safely as well.



For the purpose of this thread, however, is not to determine "something is wrong", which I think is already reasonably well established at the first post. It's to determine what the problem is and to help the OP get a correct diagnosis to actually fix the problem, whether it's by fixing it himself or getting a professional mechanic involved. There's no point in blindly replacing parts in the hopes it will fix the issue.

[Jolly Roger bus 223]

i have the same engine as the OP but mine is an 86 and his in an 89 and it doesnt have an air dryer or anything fancy not even an automatic purge valve.
belt driven compressor and the only purge valve is the manual tank drains.
thats for my bus 3 years older than the meyermobile.
so if it were me having this problem and an auto purge i never had before then i would suspect the manual drain purge valves as pointed out to have trash in them and pulled and replaced. But for meyermobile it sounds like he has an auto purge that has some trash in it or is wore out and might not hear it leaking while driving and it could be the governor is sticking and not quite shutting off at cut out pressure and once it does close it is sticking when its time to open and not regulating like it should so maybe the purge valve is fine?
just some idea,s

[Sehnsucht]

Thank you Jolly Roger for that insight into this particular model. Without an air dryer sounds even more likely to be debris, either ingested solid particles or years of moisture rotting away the tank interior meaning the debris is actually rust flakes. If this is the case I regret to say the OP has a much larger issue on their hands. I'm not scolding here but reminding that these are not vehicles like your Honda Accord that you can just gas up, change the oil, and go - diesel engines and air brake systems do require a minimal amount of attention and respect or they will let you down at the most inopportune time. It is one of the reasons that warnings like what the OP experienced are important because the responsibility of driving a heavy vehicle such as these is greater than is sometimes emphasized here. We grow comfortable and therefore complacent, expecting the safeties and backup systems to always be there until they fail us. I'm glad the OP didn't experience a catastrophe and I hope the problem isn't as severe as I have theorized, but I also hope this reminds us all not to take for granted the duty we all have to mind our rigs are in top working condition before we hit the road each day. Maybe I and Brad Swiftfur and I know others on this forum who are also truck drivers are burdened with the knowledge and perhaps memories of what happens when drivers don't respect their rigs and the gruesome consequences. I'll step off the soapbox now.

[meyermobile]

Thanks Brad,

I think it's the air Purge that is not closing completely, or is delaying.

There is a leak near the pump. Going to investigate that today and spray like crazy to find that leak! This could be linked to the intermittent governor failure.

I'll send some pics and story on my next update.

[Seriousracer]

Originally Posted by Sehnsucht

Did it pass the 4 point air brake safety check during the pre-trip inspection?

No one does that.

And next to checking the slack adjuster for correct play it IS the most critical thing to do.

[Dirtdoctor]

Exactly! Your messing with death. Yours or some other innocent people. Tow it to a truck repair shop. Airbrakes require responsibility. Soapy water spray every connection when it bubbles you’ve found a leak.

[Mekanic]

Originally Posted by Rivetboy

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....

WOW. “Emergency failure mode” ??!
I never heard that term before.

You know it sounds like his brakes work just fine. (Sounds like safety features built into modern air brake work as well) His problem is intermittent air supply.
It seems he has a quite large air leak one moment . And no leak (or one much smaller) the next.

That sounds like a prime candidate for sticking dryer valve. At least that is where I start looking.