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Old 10-22-2018, 11:29 AM   #1
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Air Break Failure

Hi All,


I have a 2005 Gillig City Bus. The Air Breaks are working great when the bus is running, but as soon as the engine is cut off, the bus begins rolling. My order of operations is, shifting the bus into neutral, pulling the break up, and shutting the engine off. I have also tried pumping the breaks down, it does not change.



Is anyone familiar with what kind of problem this would be? Have they experienced anything similar? I am assuming mechanical but I also could have something in the way I am operating that is causing this problem. Thanks for any insights.
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Old 10-22-2018, 12:23 PM   #2
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That doesn't sound right.

What do your air pressures say? As long as you have pressure, the brakes should operate with or without the engine running or even the key on.

What lever are you pulling? If it's a lever on the floor, that would likely be a cable operated park brake, something that isn't common with air brakes. Most vehicles equipped with air brakes will have a spring brake chamber that is disengaged by air pressure.

Are you sure you have air brakes?
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Old 10-22-2018, 01:19 PM   #3
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Originally Posted by Booyah45828 View Post
That doesn't sound right.

What do your air pressures say? As long as you have pressure, the brakes should operate with or without the engine running or even the key on.

What lever are you pulling? If it's a lever on the floor, that would likely be a cable operated park brake, something that isn't common with air brakes. Most vehicles equipped with air brakes will have a spring brake chamber that is disengaged by air pressure.

Are you sure you have air brakes?
Thank you for the reply. I do have normal pressure on the gauge when I'm trying this. I am pulling a yellow knob that says "pull to apply, parking break, push to release" to the left of me on the drivers dash, nothing on the floor.



(like this.)
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Old 10-22-2018, 01:59 PM   #4
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Well, that is the parking brake knob aka the spring brakes. Pulling on that should apply the spring brakes and the bus should no longer move, running or not. Pushing the knob supplies air pressure to the spring brakes counteracting the force of the spring.

So let me get this straight.

With the engine off, and your foot pushing on the brake pedal, the bus still moves?

Like I said, the engine running or not shouldn't affect the brakes as long as you have air pressure. And without air pressure, that knob should automatically come out, applying the spring brakes.
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Old 10-22-2018, 02:30 PM   #5
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Originally Posted by Booyah45828 View Post
Well, that is the parking brake knob aka the spring brakes. Pulling on that should apply the spring brakes and the bus should no longer move, running or not. Pushing the knob supplies air pressure to the spring brakes counteracting the force of the spring.

So let me get this straight.

With the engine off, and your foot pushing on the brake pedal, the bus still moves?

Like I said, the engine running or not shouldn't affect the brakes as long as you have air pressure. And without air pressure, that knob should automatically come out, applying the spring brakes.
Thank you. To clarify, with the engine off and the parking break on, the bus rolls, unless I have my foot on the floor brakes. So when I walk away to try and park the bus, it goes rolling.

With the engine on and the parking break on, the bus does not roll, whether my foot is on the floor brakes or not. So it is capable of working, it just refuses to when the engine is off.
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Old 10-22-2018, 02:31 PM   #6
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It sounds like your rear brakes are out of adjustment, and the springs can't throw the brake shoes far enough to effectively meet the drums.

That would mean you need to check and adjust the slack, though I assume your bus has automatic slack adjusters. Your brake linings and/or drums might have excessive wear. That would mean a brake job.

What you need is someone who knows how to inspect (and adjust) air brakes look at your bus.

Block your wheels if you are going to crawl under your bus and look.
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Old 10-22-2018, 02:34 PM   #7
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was your bus ever towed perhaps to the lot where you got it? it sure sounds like the rear brakes are Caged.. (as others have mentioned, the spring brakes should apply when the air tanks go empty or when you pull the parking brake lever..



if you pump your brakes dry and the bus still rolls then likely the rears are Caged..

caging is a method to releasde the rear brakes when no air can be pumped in.. ie during towing.. there are bolts which are screwed into the center of each air cylinder that force the spring brakes into release.. if the caging bolts were left in, the result would be no parking brake and no auto brakes if the pressure is drained to 0/
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Old 10-22-2018, 03:39 PM   #8
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If the knob is pulled out, the spring brakes should be applied.

You need to find a mechanic that understands air brakes and can do troubleshooting. What you're describing makes no sense.
The engine being off shouldn't affect your parking brakes.

If the brakes were caged or so far out of adjustment to cause the issue they wouldn't work regardless if the engine was running or off.

You have something odd happening that I've never seen before.
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Old 10-22-2018, 05:58 PM   #9
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Quote:
Originally Posted by Booyah45828 View Post
If the knob is pulled out, the spring brakes should be applied.

You need to find a mechanic that understands air brakes and can do troubleshooting. What you're describing makes no sense.
The engine being off shouldn't affect your parking brakes.

If the brakes were caged or so far out of adjustment to cause the issue they wouldn't work regardless if the engine was running or off.

You have something odd happening that I've never seen before.

his front brakes would work as normal even if the rear were caged.. and driving short distances in an unloaded bus the OP may not even notice the lack of rear brakes...
-Christopher
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Old 10-22-2018, 06:28 PM   #10
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Either the rear brake chambers are caged or the internal springs are broken. I have seen this many times. One spring breaks and the other still holds the vehicle. When the other one goes, then no parking brake.
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Old 10-22-2018, 08:48 PM   #11
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I see three possible scenarios here (assuming air brakes, of course).


1. (unlikely) Parking brake was not set by driver. I think we've established that you have pulled the knob (on some really old models they worked the opposite way by pulling out to drive and pushing in to park.)


2. Brakes out of adjustment, beyond wear limits, defective slack-adjusters, or cammed-over. Excessive brake wear can and eventually will result in brake "Cam-over", after which *YOU HAVE NO BRAKES AT ALL!* Not parking brakes, nor service brakes. You *NEVER* want this to happen! This is a catastrophic failure and usually results in major crashes. I cannot stress this enough. This is why regular inspections are required, minimum brake thicknesses (typically 1/4" or so in the middle of the shoe is the minimum) and brake drum maximums.


3. Brakes are caged or broken springs. Caging is usually done for towing. It will disable the parking function, *BUT*, the service side *STILL WORKS!* This is also done for brand-new trucks piggy backed to dealers. The same holds true with broken springs (often this may damage the diaphragm resulting in air leaks, but not always).


If your service brakes work fine (all of them, not just the fronts) but pulling the parking brake does not keep the bus in one place, I'd check for #3. Test this by trying to drive the bus with the knob in the "parked" position (assuming it will let you), or pulling the knob while moving very slowly (do this in a safe and controlled place; it *SHOULD* lurch to a stop). As noted above, caged brakes are held in "released" position by bolts made for the purpose. Remove them carefully and remember they are retaining a *LOT* of energy. Store them in the tubes on the brake chambers made for the purpose. If the brakes aren't caged, but still refuse to hold the bus still, I'd suspect broken springs (somewhat common after 10 years and who knows how many uses).



Air brakes really aren't that complex and once you understand how they work, it's fairly simple to determine what's wrong with them.
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Old 10-22-2018, 10:05 PM   #12
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Thanks all, I appreciate the advice everyone is sharing a lot. I am working on having an air break mechanic come out to work on it.


It is likely it was towed at some point recently due to it being from FL. Though, I tested it again, and can confirm that the air break works when the engine is on. I can get out of the bus, leave it running and it doesnt go anywhere. Once I crank off the engine in the same spot, it rolls back onto the bricks again.
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Old 10-22-2018, 11:31 PM   #13
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Whatever you do, DO NOT try to disassemble the rear spring brakes. If you undo the clamp around them, they'll explode with enough force to maim or kill you. It sounds like you need an air brake tech to check them over, thoroughly. Driving a heavy vehicle without correctly functioning brakes will put you at great legal risk if you are in any accident.

John

PS - Bendix has lots of useful info about air brakes. Read it!
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Old 10-23-2018, 02:11 AM   #14
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Quote:
Originally Posted by gillig_streets View Post
Thanks all, I appreciate the advice everyone is sharing a lot. I am working on having an air break mechanic come out to work on it.


It is likely it was towed at some point recently due to it being from FL. Though, I tested it again, and can confirm that the air break works when the engine is on. I can get out of the bus, leave it running and it doesn't go anywhere. Once I crank off the engine in the same spot, it rolls back onto the bricks again.

Many newer buses have safety interlocks designed to keep the bus from moving while the doors are open. Pulling the parking brake should result in the bus *STAYING PUT NO MATTER WHAT*. I think it's safe to say your brakes may still be caged or needing repair.


Now, as for it staying put while the engine is running, it is likely to have another safety feature that applies the service brakes with the door open/transmission in neutral. It is assumed that when the bus is actually parked, the parking brake is used and this safety feature is not needed with the engine off. (It's also a federal mandate. Air pressure can and usually does leak down with the engine off, and parking brakes *MUST* be of a design that does not rely on air/hydraulic pressure which can leak away, which is why they use springs.)
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Old 10-23-2018, 02:33 AM   #15
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Originally Posted by Iceni John View Post
Whatever you do, DO NOT try to disassemble the rear spring brakes. If you undo the clamp around them, they'll explode with enough force to maim or kill you. It sounds like you need an air brake tech to check them over, thoroughly. Driving a heavy vehicle without correctly functioning brakes will put you at great legal risk if you are in any accident.

John

PS - Bendix has lots of useful info about air brakes. Read it!

John has it right. Brake chambers of old had 2 clamps holding them together. One secured a spring under a *LOT* of pressure; the other held the service side together. These days (the last 20 years or so) the spring side of the chamber is pressed together, the other side still uses clamps so the chamber can be partially replaced (the section with the spring and diaphragms). I have to agree, though. *DO NOT* disassemble this unless you know what you're doing as it still has pressure on it. The only thing I recommend novices doing (and even this is with great care) is removing the caging bolts - and I won't fault you for calling in an expert on this. Brakes are perhaps the most important thing on any vehicle and I think we all agree, we all want to know our brakes work the way they are supposed to.
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Old 10-23-2018, 08:30 AM   #16
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I must not be able to read.

With the engine on, and the knob pulled out, the bus doesn't move, Correct?

With the engine off, and the knob pulled out, the bus will move, Correct?

What about if the key is on, and the engine is off, what does it do then?

With the engine running, shut the door and see if it will roll?

Does the bus have a handicap lift on it by chance?

Your spring brake should never disengage by shutting the engine off and then re-engage with the engine running.

Maybe, your bus has a safety interlock that engages the service brakes with the door open. Turning the key off will turn the safety interlock off, which is why it will then roll. That combined with your rear brakes being caged or inoperative could cause what your experiencing.

I've only seen that type of system used on busses with handicap lifts and it was only in play with the handicap door open. The front door, however, didn't affect it. Maybe being a city bus they use this system on all doors. Anybody work on a city bus before and can comment?
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Old 10-27-2018, 04:45 PM   #17
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GetYour bus to a air brake shop. It would be tragic if your vehicle was in an accident and you were to fault due to ignorance.
Get them checked and repaired so you won’t give all the other skoolies a bad name.
Getter fixed.
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Old 10-29-2018, 08:12 PM   #18
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Although that looks like a park button for airbrake it also resembles the park brake/release for a hydraulic brake system used some years ago that would explain the manual park brake described in op. Is there an airpressure gauge?
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Old 11-19-2018, 10:33 AM   #19
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rear brakes

Quote:
Originally Posted by gillig_streets View Post
Thanks all, I appreciate the advice everyone is sharing a lot. I am working on having an air break mechanic come out to work on it.


It is likely it was towed at some point recently due to it being from FL. Though, I tested it again, and can confirm that the air break works when the engine is on. I can get out of the bus, leave it running and it doesn't go anywhere. Once I crank off the engine in the same spot, it rolls back onto the bricks again.
do not drive the bus anymore until the mechanic looks at your brakes, these guys know what their stuff, your front brakes are doing all the work, they will fade, and you will be on the news, i have rebuilt air brake systems, unless you took a brake system a cozad heavy duty trailer, your rear brakes are messed up. PLEASE do not drive this bus until fixed.
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