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    Default Air Brake System Question

    First off, I don't know squat about air brake systems, so maybe there's an obvious answer to this issue. If so, great.

    We just recently had air lines run from a central air compressor in our station to our two trucks that have air brakes. The regulator coming off of the central compressor is set at 80 psi. When we plug this shop air into the air connection on our engine, we can hear air leaking out from under the truck. The air seems to be coming from a little widget attached to the bottom of the air dryer that has two wires coming out of it (a search of the Bendix parts system indicates this is a purge valve). It only leaks air through this purge valve when plugged in to shop air. When we were messing with it, the air pressure in the truck was over 100 psi, so I wouldn't think that there should have been any air flow from the shop air system into the truck. But, like I said earlier, I don't know enough to know anything.

    If anyone can explain why this might be happening, I'd appreciate it. The air system on the truck in question holds air pretty well, so we've just left it unplugged for now.

    Thanks in advance,

    Andy

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    Since an air brake work opposite of hydraulic brakes, a steady psi of air is not required to keep the parking brake set. Hell the system can have Zero psi and the brake will stay set. This would be the only reason I would think of that the purge valve expels air while parked, the extra air is not needed.

    I have not seen a rig that had its air brake's tanks hooked to an external pump in years. (1998 to be exact). What is the year and make of this apparatus?

    I dunno, I just know how to operate them, so that's all an educated guess.

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    I would like to have "Chief Engineer" chime in on this, but from my perspective, your mechanic connected the shop air into the wrong place in the system. When you shut off the truck, the purge valve is supposed to dump the air from the air drier and the compressor so it doesn't start under load and any water condensed in the air drier is dumped out on the floor. There is a check valve between the purge valve and the tank, and the shop air needs to be hooked in after the check so it keeps the tank full.

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    Default Understanding Air Brakes

    Take a look at this web site. http://www.gnb.ca/0276/vehicle/pdf/ab_manual-e.pdf On page 15 it shows a diagram for the air drier and describes its function. In the drawing at the lower right is the check valve. I suspect that your mechanic made the hook-up for the shop air between the compressor and the air drier. It should have been after the air drier.

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    Quote Originally Posted by KuhShise View Post
    Take a look at this web site. http://www.gnb.ca/0276/vehicle/pdf/ab_manual-e.pdf On page 15 it shows a diagram for the air drier and describes its function. In the drawing at the lower right is the check valve. I suspect that your mechanic made the hook-up for the shop air between the compressor and the air drier. It should have been after the air drier.
    Given the OP's description, I concur with this evaluation. Let the Chief Engineer confirm it.
    "Loyalty Above all Else. Except Honor."

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    We do use that setup on all of our air braked vehicles, even our newest one. All of ours are plumbed directly into the first or "wet" tank, through a check valve. Doing so does bypass the air dryer. When hooking up that way it's important to have a good air drying system on your station compressor or you'll be putting condensate from the station compressor into vehicle's air system.

    If you don't have a dryer on the station system you probably want to invest in one. Otherwise, you'd have to connect so the air passes through the vehicle's dryer. That would take some trickier plumbing including a couple of check valves, and it would shorten the life of the dessicant in your vehicle dryer.

    As to your air leak while plugged in, a possibilties. The check valve in the vehicle air dryer may be in need of replacement. The only dryer that I've ever personally had apart (several times) is the Bendix AD-4. A very common, good unit buf they were notorious for having check valve problems. I'm pretty sure that one's been replaced by later models. The check valve for the AD-4 can be bought separately and is not expensive.

    If it hasn't been done in a long time you might want to consider doing a complete rebuild including a dessicant pack. But if you aren't into truck mechanicking, send it out. I'm not sure if exchange units are available for them or not. Exchange meaning you get a factory rebuilt unit and trade your old one in. If they're done by B/W or BEPCO, they're every bit as good as brand new, and for less money.

    Something you might try in order to get a better handle on your leak. Drain down your vehicle air system, then start it up. Have someone lie under it and feel the valve where you get the air leak. See if it leaks anytime during the build up of air from the vehicle compressor. If it does, at what pressure does it begin to leak. Also note how long it takes to build up air, or if it has any difficulty coming up to full pressure. All signs of a bad check valve.

    One question - you mention having two vehicles plugged into your station system. Do you get this from both of them, or just one? If you're getting it from both, you might want to consider bringing the house system up to 100 or 110 psi and see what that does.

    Not to pass the buck here, but I'd like to get a second opinion from FIREMECH1 on this one.

    Stay safe out there, everyone goes home!

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    Quote Originally Posted by KuhShise View Post
    Take a look at this web site. http://www.gnb.ca/0276/vehicle/pdf/ab_manual-e.pdf On page 15 it shows a diagram for the air drier and describes its function. In the drawing at the lower right is the check valve. I suspect that your mechanic made the hook-up for the shop air between the compressor and the air drier. It should have been after the air drier.
    Thanks, I actually found that same diagram last night and I'm wondering if the connection somehow got made in the purge valve control port line. I'll have to do some snooping around when I get back in town.

    Quote Originally Posted by chiefengineer11
    One question - you mention having two vehicles plugged into your station system. Do you get this from both of them, or just one? If you're getting it from both, you might want to consider bringing the house system up to 100 or 110 psi and see what that does.
    We're only having the problem on one of the trucks. The other one seems fine. I do want to check the plumbing on the other truck also though, since I'm not sure where the external connection is hooked in. I don't think the shop air system has a dryer on it right now, so that might be an issue. This is what happens people try to get things "for free". They end up costing you more than it would have cost to put a Kussmaul compressor in each truck

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    Quote Originally Posted by FiremanLyman View Post
    Since an air brake work opposite of hydraulic brakes, a steady psi of air is not required to keep the parking brake set. Hell the system can have Zero psi and the brake will stay set. This would be the only reason I would think of that the purge valve expels air while parked, the extra air is not needed.
    Right, the reason to plug a truck in to shop air is so that if it has a leak in the air system (and most do somewhere), you'll have have air and be ready to roll even if the truck has been sitting for a few days, instead of having to let it run for a few minutes to build up enough pressure to disengage the brakes.

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    Sorry for the delay.

    From the sounds of it, someone teed into the line between the governor and the control port on the drier. With that done, air is escaping through the purge valve. More than likely, depending on the year of the truck, it should have an AD-9 drier on it.

    If you must hook up station air, tie it directly into the primary/wet tank. If possible, get another AD-9 and plumb it as close as possible to the station compressor. This will help in keeping any moisture from it, from settling in the trucks air tanks and system.

    The 2 wires on the purge valve are for the heater on it. It keeps the purge valve from freezing open when it hits cut-out, or what I call "sneezes".

    FM1

    EDIT: I wouldn't recommend plumbing the station air through the trucks drier. It will shorten its life considerably.
    Last edited by FIREMECH1; 12-30-2009 at 01:54 PM.
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    Quote Originally Posted by FIREMECH1 View Post
    If you must hook up station air, tie it directly into the primary/wet tank. If possible, get another AD-9 and plumb it as close as possible to the station compressor. This will help in keeping any moisture from it, from settling in the trucks air tanks and system.
    Hadn't thought of that, that would probably be cheaper than some of the shop air dryers I've seen. What would the control port of the AD9 be plumbed to in that situation? I'm assuming that would needed in order to periodically purge the dryer of moisture?

    I know Bendix recommends a rebuild of the air dryer every 3 years or 300,000 miles, but we barely put any miles on the truck so I would think we could stretch past 3 years a bit. The truck in question was built in 2000, so it's probably time for a rebuild (I'm guessing it's never been done, I'll have to have the records checked).

    Thanks for all of the info, I'm learning a lot.

    Andy

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    You can fabricate a small holder for the AD-9 so it will stand upright on the floor. Then connect a line from the station air compressor to the "in" port, and then run from the "out" port, your air line to your wet/primary tank. Put a date on the canister, and then replace it every other year, also dating them.

    All other openings should be plugged. As for the purge valve operation, don't worry about it, as it really doesn't need to be operational for what we/you are doing.

    @ Chief11... Sorry for the short, nondescript reply. As well, your post was well done.

    @ Kuh... Thanks for the heads up. Much appreciated.

    FM1
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    Quote Originally Posted by EastKyFF
    "Firemens gets antsies. Theys wants to goes to fires. Sometimeses they haves to waits."

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    Quote Originally Posted by KuhShise
    When you shut off the truck, the purge valve is supposed to dump the air from the air drier and the compressor so it doesn't start under load and any water condensed in the air drier is dumped out on the floor.
    Not exactly true my friend.

    The only time the purge valve opens is when the governor has hit it's cut off/out psi (usually around 120psi), just after the governor tells the compressor it has enough air pressure. When the air system hits 120psi, it unloads the compressor, and at the same time, there is a pressure differential on the purge valve. When it sees this difference, it allows air from the wet/primary tank to reverse, and expel moisture, oil, and other contaminants from the system through the purge valve. It also allows that dry air to recharge the dessicant in removing moisture.

    Try this as an example. Bleed your air system down to below 70psi, then start the rig up and watch your air pressure increase. As soon as it hits 90-100 psi, shut the rig down. You will notice that the purge valve didn't open, or as I call it "sneeze". The reason it didn't is because the governor didn't hit max psi, and tell the compressor to shut down.

    Remember physics 101... you can compress air. So starting a truck with a load on the compressor/governor won't hurt anything.

    FM1
    I'm the one Fire and Rescue calls, when they need to be Rescued.

    Quote Originally Posted by EastKyFF
    "Firemens gets antsies. Theys wants to goes to fires. Sometimeses they haves to waits."

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    Smile The More you learn!!!

    Thanks Firemech: I should have realized what was happening after looking at the diagrams. Anyway, this just re-enforces my oft repeated statement.... The more you learn, the dumber you find out you are!! Or said another way, never, NEVER stop learning. There is always something that you don't know and might need to. Sometimes you just need to figure out what it is that you don't know and then keep your Yap shut!! Thanks for taking the time to correct my misinformation.

    Kuh
    Last edited by KuhShise; 12-31-2009 at 02:52 PM.

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    I'll add a couple of things. First I would bring the pressure up on the regulator to 115 or so. That way the truck is pretty well completly aired up when you start it.

    The other thing is your air system in the station. All of our stations have the regulator on the ceiling so it can not be "adjusted" by someone. All of the air outlets other than for trucks are on a seprate line. The regulators are set at 115 psi and the outlets are set at 150 psi. Another thing we do is each truck drop has a valve on it, that way if something happens and a drop get broke we can just turn it off and the rest of the system stays charged.

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    Quote Originally Posted by KuhShise
    Sometimes you just need to figure out what it is that you don't know and then keep your Yap shut!! Thanks for taking the time to correct my misinformation.
    Kuh
    This is why you and I are here, and everybody else. To get and share the right information/help.

    Heaven knows, you guys have corrected or helped me in all phases of what you do. And I appreciate that.

    FM1
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    Quote Originally Posted by EastKyFF
    "Firemens gets antsies. Theys wants to goes to fires. Sometimeses they haves to waits."

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    Quote Originally Posted by FIREMECH1 View Post
    Sorry for the delay.

    From the sounds of it, someone teed into the line between the governor and the control port on the drier. With that done, air is escaping through the purge valve. More than likely, depending on the year of the truck, it should have an AD-9 drier on it.

    If you must hook up station air, tie it directly into the primary/wet tank. If possible, get another AD-9 and plumb it as close as possible to the station compressor. This will help in keeping any moisture from it, from settling in the trucks air tanks and system.

    The 2 wires on the purge valve are for the heater on it. It keeps the purge valve from freezing open when it hits cut-out, or what I call "sneezes".

    FM1

    EDIT: I wouldn't recommend plumbing the station air through the trucks drier. It will shorten its life considerably.
    Can you tell me the difference between and AD-8 and AD-9? We run AD-8s, had 1 AD-IP that was a constant problem so I replaced it with another AD-8. I saw an AD-9 at a local Peterbilt dealer and it looked like the same configuration as a AD-8, just more capacity. I asked the parts dept who had it on display and they couldn't tell me the difference between them.

    Thanks for the idea of setting up an AD-8/9 air dryer in line at the firehouse. I've had in-line air dryers that just didn't keep up, the AD-8/9 will be a big improvement. Is there any way to adjust the cut-out pressure of the AD-8/9?
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    Quote Originally Posted by DFDMAXX
    Can you tell me the difference between and AD-8 and AD-9? We run AD-8s, had 1 AD-IP that was a constant problem so I replaced it with another AD-8. I saw an AD-9 at a local Peterbilt dealer and it looked like the same configuration as a AD-8, just more capacity. I asked the parts dept who had it on display and they couldn't tell me the difference between them.
    To be honest, I have never heard of an AD-8. Outside of the AD-9's and AD-IP/SP, there isn't one that I know of.

    Quote Originally Posted by DFDMAXX
    Thanks for the idea of setting up an AD-8/9 air dryer in line at the firehouse. I've had in-line air dryers that just didn't keep up, the AD-8/9 will be a big improvement. Is there any way to adjust the cut-out pressure of the AD-8/9?
    The only way to control or adjust the cut-out pressure is through the governor.

    FM1

    EDIT: I'm not sure who is more confused, you or me???
    Last edited by FIREMECH1; 01-03-2010 at 05:01 AM. Reason: Do I need a reason???
    I'm the one Fire and Rescue calls, when they need to be Rescued.

    Quote Originally Posted by EastKyFF
    "Firemens gets antsies. Theys wants to goes to fires. Sometimeses they haves to waits."

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    Quote Originally Posted by FIREMECH1 View Post
    To be honest, I have never heard of an AD-8. Outside of the AD-9's and AD-IP/SP, there isn't one that I know of.



    The only way to control or adjust the cut-out pressure is through the governor.

    FM1

    EDIT: I'm not sure who is more confused, you or me???
    Uhm........ dunno. I'd swear it's an AD-8, looks just like an AD-9 only shorter. BUT.... It wouldn't be the first time I was wrong either. I'll go crawl under and look.

    My question about adjusting the cut-out pressure was in how it would work with the station compressor as an in-line dryer for the station. Should I adjust the station pressure to match the dryer cut-out so it exhausts when it is supposed to?
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    I did a little digging under the truck today and it appears that the shop air on the troublesome truck is plumbed in between the compressor and air dryer, right at the air dryer supply (in) port. So apparently it is plumbed to run the shop air through the truck's dryer. A few people have already said this is bad because it would cause premature dryer failure, so I'd to follow up on that a bit.

    Is this decreased dryer life due to the fact that if there is a leak, we'd be constantly running air through the dryer 24/7? If so, and if the truck in question holds air well (i.e. no or very very slow leak), would this decreased dryer life be a non-issue?

    Regardless of that issue, I'm still a little confused as to why the purge valve opens when shop air is plugged into that location. I don't see why that happens. Also, it doesn't happen regularly. I wonder if it has something to do with the position the governor happens to be in?

    Andy

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    Quote Originally Posted by Pelican631 View Post
    I did a little digging under the truck today and it appears that the shop air on the troublesome truck is plumbed in between the compressor and air dryer, right at the air dryer supply (in) port. So apparently it is plumbed to run the shop air through the truck's dryer. A few people have already said this is bad because it would cause premature dryer failure, so I'd to follow up on that a bit.

    Is this decreased dryer life due to the fact that if there is a leak, we'd be constantly running air through the dryer 24/7? If so, and if the truck in question holds air well (i.e. no or very very slow leak), would this decreased dryer life be a non-issue?

    Regardless of that issue, I'm still a little confused as to why the purge valve opens when shop air is plugged into that location. I don't see why that happens. Also, it doesn't happen regularly. I wonder if it has something to do with the position the governor happens to be in?

    Andy
    The issue with running your station air through the vehicle dryer is that you risk shortening the life of your dessicant pack. But if you don't have a drying system on your station system and you bypass the on board dryer, you are dumping condensate and whatever else is in your station air directly into your vehicle tanks. That, in turn, partially defeats the purpose of the on board dryer. If you have a tight vehicle system the potential for shortening the life of the on board dryer is less of an issue, but it doesn't go completely away.

    The best solution is to put a dryer/filter on your station system. As has been suggeeted here, you can use an AD 4, 9, or whatever, or you can buy a stationary system dryer. Before making that decision I'd price out each and try to determine their projected life between service intervals.

    Certainly one of the truck dryers would work, but they have stuff in them you don't normally need in a stationary system (purge valve, check valve, etc), and could cost you more in the long run.

    Talk to a truck parts house for an on board type unit. There are units that compete with the Bendix units. The truck fleet that I came from would not consider any others, though. Also, no matter what you get, make sure it is a dessicant type rather than a coalescing type. Coalescors will not work without air flowing over them.

    Some fire truck builder was using coalescors once. They wanted us to use it instead of a dessicator. I said, "No," because where they would have mounted it, there would have been poor air flow. They work very well on the outside of a tractor frame rail, but not under the body of a fire truck.

    Talk to a service station equipment dealer or body shop equipment dealer about stationary systems. Compare notes. Once you have that taken care of, plumb the vehicle to the wet tank.

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    Quote Originally Posted by DFDMAXX
    My question about adjusting the cut-out pressure was in how it would work with the station compressor as an in-line dryer for the station. Should I adjust the station pressure to match the dryer cut-out so it exhausts when it is supposed to?
    No, you shouldn't have to adjust anything. With that, I'll post up a diagram of what you will need to make it work, as it does on the rig. I've set one up last year with a governor, and it works fine.

    My problem is time right now, and the reason for sporadic responses. I've been working 14 hour days since Dec 6 due to the onslaught of snow then, and lately, and being predicted. I still haven't done Christmas with the kids and grandkids yet.

    FM1
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    Quote Originally Posted by EastKyFF
    "Firemens gets antsies. Theys wants to goes to fires. Sometimeses they haves to waits."

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    Quote Originally Posted by FIREMECH1 View Post
    No, you shouldn't have to adjust anything. With that, I'll post up a diagram of what you will need to make it work, as it does on the rig. I've set one up last year with a governor, and it works fine.

    My problem is time right now, and the reason for sporadic responses. I've been working 14 hour days since Dec 6 due to the onslaught of snow then, and lately, and being predicted. I still haven't done Christmas with the kids and grandkids yet.

    FM1
    Thanks for any help you can give, a diagram would be great. No rush.
    We do not rise to the occasion. We fall back to our level of training.

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    Quote Originally Posted by FIREMECH1 View Post
    To be honest, I have never heard of an AD-8. Outside of the AD-9's and AD-IP/SP, there isn't one that I know of.



    The only way to control or adjust the cut-out pressure is through the governor.

    FM1

    EDIT: I'm not sure who is more confused, you or me???
    Uhm...... yeah.... I was wrong. AD-9. Not sure where I got the 8 from, but at least I screwed up on an international public forum! No... wait... that's bad too..

    Sorry about the confusion.
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    Talking Air Dryers

    DFDMAXX:
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