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Old 12-13-2019, 10:31 AM   #1
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Air Brake Woes

I started out addressing leaks. Now it appears I have governor issues. Leaks I can figure out myself. The governor is what I hope for input on. Including full history of problems/fixes if it helps, but TL;DR to 'MY PROBLEM' in red:

History of problems & fixes: PP-DC push-pull (parking brake) valve has leaked from the time we got the bus, but everything else appeared to work properly. Bus was brought home & parked, only moving to haul seats off to the salvage yard.

Next (first time I noticed it was at the salvage yard) the discharge from air dryer @ cutout pressure became 'prolonged'. Not a quick, sharp release, but a long, slow release, that eventually trails off to stop. Cutout is at the proper pressure (~120psi). Compressor did stop building pressure as expected at cutout, & did cut back in as expected as well.

Replaced the PP-DC valve & checked for leaks. All good. But now I've got leaks at the foot valve. Replaced foot brake fittings & tested. All good. But now I've got leaks from the line feeding the door/seat. Replaced & tested. All good, but now I've got a leak from one of the large lines feeding through the bulkhead (Not fixed yet, that was yesterday).

MY PROBLEM... Yesterday on my leak hunt the compressor stopped cutting back in. The first time I had run the engine up to cut-out (120 psi, still with long, slow discharge from air dryer), but once pressure got down to cut-in (leaks + brake application), nothing happened. Let the pressure drop to below 40 psi or so while the engine ran to see if it was going to cut back in at all (which it did not), & then I shut off the engine & went to look under the hood. Came back about 5 minutes later (remaining pressure had completely bled off by now from aforementioned leak), started engine back up, and it once again built pressure up to cut-out as normal. Foot-pumped back to cut-in and nothing again... just kept draining.

Obviously I have governor issues, but do the symptoms sound like other issues as well. Also, would you think this is just a failed governor, or something external to the governor impacting it (control line?). Finally, would the odd air-dryer behavior be consistent with a failed governor, and/or leaks in the air system, or do you think that's a problem in & of itself?
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Old 12-13-2019, 10:59 AM   #2
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The drier has a check valve between it and the wet tank. Those fail and will cause a prolonged air purge from the drier, It will also cause frequent cycling of the governor/drier purge.

Governors are cheap, replace it if you think it's bad, then check drier check valve.

I can give you instructions on how to check it if needed.
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Old 12-13-2019, 11:10 AM   #3
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Thanks so much, Booyah! Much appreciated. I'll go ahead & replace the governor, & go from there. If I still have problems I may take you up on that offer to explain how to check that valve.

Are there any common causes of premature governor failure external to the governor itself? Not that this is necessarily premature. Have no idea how old it is. But I don't want to replace the governor alone when a potential problem exists elsewhere.
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Old 12-13-2019, 11:13 AM   #4
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Thanks so much, Booyah! Much appreciated. I'll go ahead & replace the governor, & go from there.


Are there any common causes of premature governor failure external to the governor itself? Not that this is necessarily premature. Have no idea how old it is. But I don't want to replace the governor alone when a potential problem exists elsewhere.
Governors wear out through use. So if you've been frequently cycling it for awhile, it will wear them out.

Based off of you replacing the other stuff because of leaks, I'd say you're in a climate that eats rubber parts, so the governor is just one more thing that's wore out.
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Old 12-13-2019, 11:28 AM   #5
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Goveners fail often and unexpectedly usually on the coldest most miserable day of the year. I carry a spare.
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Old 12-13-2019, 01:12 PM   #6
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Thanks guys! A spare sounds like a sound option, so I'll order two when I do!


Booyah, the leaks are as received. Bus was purchased at auction in Austin TX in summer of this year. From what I can tell, the leak at the push-pull was so bad the others weren't apparent. Each time I fix one a new one pops up. Before I'm done we might have an all-new brake system.
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Old 01-08-2020, 06:03 PM   #7
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Well, making progress. Governor is replaced, & a couple more leaks addressed. Compressor is cutting in & cutting out as I'd expect it to. However the extended purge from the air dryer is still present. In fact, it now purges right until cut-in. So...


1) pressure builds up from 0 to ~120 psi as expected.
2) compressor stops building pressure & air dryer starts purging
3) air dryer never stops purging until right around cut-in pressure (then does stop)
4) compressor builds up to ~120 psi, & cycle starts over.

Booyah, you mention a check valve between the wet tank & dryer. Did you mean between the dryer & primary/secondary supply tanks? I don't have a wet tank, just copper from the compressor to a junction where it turns to something softer, and that runs directly to the supply port on the dryer (AD-9). On the delivery side, however, there's a check valve that screws into the dryer, and that line runs to the secondary side of the combined primary/secondary storage tanks.
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Old 01-09-2020, 09:44 AM   #8
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Well, making progress. Governor is replaced, & a couple more leaks addressed. Compressor is cutting in & cutting out as I'd expect it to. However the extended purge from the air dryer is still present. In fact, it now purges right until cut-in. So...


1) pressure builds up from 0 to ~120 psi as expected.
2) compressor stops building pressure & air dryer starts purging
3) air dryer never stops purging until right around cut-in pressure (then does stop)
4) compressor builds up to ~120 psi, & cycle starts over.

Booyah, you mention a check valve between the wet tank & dryer. Did you mean between the dryer & primary/secondary supply tanks? I don't have a wet tank, just copper from the compressor to a junction where it turns to something softer, and that runs directly to the supply port on the dryer (AD-9). On the delivery side, however, there's a check valve that screws into the dryer, and that line runs to the secondary side of the combined primary/secondary storage tanks.
That's typical operation for when the drier check valve fails.

The check valve is at the outlet of the drier. I wouldn't consider it external to the drier assembly, because it comes as part of the drier assembly, but you can replace it with very little disassembly. It is haldex part number 107800k for an ad9 drier. I've seen some places thread in an additional external check valve after the internal valve when the internal one fails, to get the bus back in service.

Here is the installation instructions for the drier. http://beamalarm.com/foretravel-link...stallation.pdf
You can see on the diagram present on page 2 that the check valve is at the bottom right corner. To give you an idea of what to look for.

I'd be surprised if you don't have a wet tank. Most of those combo tanks have a wet, secondary, and primary chamber internal in the one tank. How many petcocks are on the bottom of it?
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Old 01-09-2020, 11:02 AM   #9
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As always, thanks so much, Booyah. I owe you a suitable container of whatever you find refreshing if/when we ever meet in person.

I pulled down the AD9 manual, so I know what you're talking about, & already have one on order. Was going to get the rebuild kit but the entire valve was just a few dollars more.

As for the tank, there are two drain petcocks total. The line from the top of the compressor plumbs directly into the air dryer (goes from copper to flexible hose, then into the dryer), and a 5/8 plastic hose comes off the air dryer (from the port where the check valve is located) and connects the the end of the tank. Does this confirm I have a combo tank including a wet tank as you describe? I thought (keeping in mind I don't know sh*t ) that the wet tank came before the air dryer. Still trying to wrap my head around the whole air brake system.


EDIT: Looking at my tank & the Bendix docs & it looks like it's exactly as you describe. The only diff is they use the term 'primary tank' in their diagrams. I can see now where the portion of the tank the dryer plumbs into is connected to two other sections (primary & secondary) through external plumbing. The term 'wet' is throwing me off. Why is it called wet if it's after the air dryer?
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Old 01-09-2020, 11:26 AM   #10
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As always, thanks so much, Booyah. I owe you a suitable container of whatever you find refreshing if/when we ever meet in person.

I pulled down the AD9 manual, so I know what you're talking about, & already have one on order. Was going to get the rebuild kit but the entire valve was just a few dollars more.

As for the tank, there are two drain petcocks total. The line from the top of the compressor plumbs directly into the air dryer (goes from copper to flexible hose, then into the dryer), and a 5/8 plastic hose comes off the air dryer (from the port where the check valve is located) and connects the the end of the tank. Does this confirm I have a combo tank including a wet tank as you describe? I thought (keeping in mind I don't know sh*t ) that the wet tank came before the air dryer. Still trying to wrap my head around the whole air brake system.


EDIT: Looking at my tank & the Bendix docs & it looks like it's exactly as you describe. The only diff is they use the term 'primary tank' in their diagrams. The term 'wet' is throwing me off. Why is it called wet if it's after the air dryer?
I appreciate that, but there's no need. I'm here to help, and this stuff can be complicated if you've never dealt with it. Just pass the favor on bud!

The copper line is normal. Compressing air makes it hot, so they run copper for the first section because it's more durable/resistant to the heat then rubber or nylon would be.

The wet tank will be directly after and fed by the drier, if one is present. Driers aren't always 100% efficient, so some moisture and oil can still end up in the wet tank. Back in the days before air driers were common, they would catch some water and be wet. Now, It's just a term they use for it, it's essentially a feed/distributoion/accesory tank.

I'd say you have some sort of combo tank, being that there is 2 petcocks on it. Do you only have one tank on the bus? Is there a spot for a 3rd petcock that somebody might have plugged?

Having only two tanks is just not a typical arrangement for a vehicle with air brakes. It'd be interesting to see how they plumbed between the two, because by law the secondary and primary systems have to be separated and protected from each other. I'm not sure how they could do that and be legal with one feeding the other.
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Old 01-09-2020, 11:45 AM   #11
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I appreciate that, but there's no need. I'm here to help, and this stuff can be complicated if you've never dealt with it. Just pass the favor on bud!

The copper line is normal. Compressing air makes it hot, so they run copper for the first section because it's more durable/resistant to the heat then rubber or nylon would be.

The wet tank will be directly after and fed by the drier, if one is present. Driers aren't always 100% efficient, so some moisture and oil can still end up in the wet tank. Back in the days before air driers were common, they would catch some water and be wet. Now, It's just a term they use for it, it's essentially a feed/distributoion/accesory tank.

I'd say you have some sort of combo tank, being that there is 2 petcocks on it. Do you only have one tank on the bus? Is there a spot for a 3rd petcock that somebody might have plugged?

Having only two tanks is just not a typical arrangement for a vehicle with air brakes. It'd be interesting to see how they plumbed between the two, because by law the secondary and primary systems have to be separated and protected from each other. I'm not sure how they could do that and be legal with one feeding the other.

Booyah covered it pretty well. I cannot remember any air-brake truck or bus (newer than 70's era) I have ever driven that didn't have 3 air tanks/compartments, or some combination thereof. I recall one had what looked like a 7-10 gallon air tank with 3 compartments (complete with 3 petcocks, and you could see the metal had been welded from the inside to form 3 chambers) and external plumbing, check-and-safety valves to bring it up to spec. Many "dual tank" setups often have one tank partitioned for "Wet" and "Primary" sections. There's no law that I am aware of prohibiting "one air tank", as long as it is properly sectioned off and has the appropriate valves and plumbing.
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Old 01-09-2020, 12:26 PM   #12
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Thanks guys. Think I got it figured out. I found the missing petcock. It's an air chuck. Now I understand the layout of the tank. I was all stoked I was getting nothing out of the drains. Now I'm fearful the wet tank is a fish bowl Think I'll replace that with a petcock & move the air chuck up to that unused bung (as seen in second pic)

The plumbing from wet to primary/secondary has me confused. It looks like they are all just plumbed together, but that's obviously not supposed to be the case. Am I correct in assuming the second pic shows a check valve? I thought at first it was just a 90-elbow. Not shown in the pic at the far end of the tank (further to the right) is an identical fitting where the line plumbs into the primary side of the tank.
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Old 01-09-2020, 01:55 PM   #13
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You figured it out!

That quick connect is supposed to be a petcock to drain the wet tank. It might not be a fishbowl, being in Arizona and having a drier, it might be dry.

The elbows going into the primary and secondary tanks are check valves separating/protecting them from the wet tank.

Those check valves are why your constant compressor cycling doesn't show up on the dash gauges. If you'd put a gauge in that air chuck, you could see wet tank pressure build up to compressor cut out, and then slowly drop back to cut in.

You can put that chuck in any unused spot on the wet tank. You could probably even put it in the secondary tank if you wanted to. Never put it in the primary side, that's where most of your braking pressure comes from, and you don't want to add anything that could compromise that.
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Old 01-09-2020, 02:25 PM   #14
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Got it. However...

I have one more question (sorry!)

My next task was to figure out what this was, & I just did. Apparently a DV-2 automatic drain valve. Is this effective in the location where it's mounted? I'd think it wouldn't do any good unless pulling from the very bottom of the tank. According to the manual it has inputs both on the top (not used currently), and the side (currently being used). Would it make sense to just swap this with the air chuck, using the top port and plugging the side? Or leave it alone & replace the chuck w/ a petcock as planned?
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Old 01-09-2020, 02:37 PM   #15
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Got it. However...

I have one more question (sorry!)

My next task was to figure out what this was, & I just did. Apparently a DV-2 automatic drain valve. Is this effective in the location where it's mounted? I'd think it wouldn't do any good unless pulling from the very bottom of the tank. According to the manual it has inputs both on the top (not used currently), and the side (currently being used). Would it make sense to just swap this with the air chuck, using the top port and plugging the side? Or leave it alone & replace the chuck w/ a petcock as planned?
If you have one of those, I'd leave it all alone. I'd say somebody installed that valve, and in the process, removed the petcock and installed the air coupler in it's place.

You won't get the tank completely empty with where the valve is at, but I wouldn't fret over it. If that valve isn't leaking air, I'd leave it be.
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Old 01-09-2020, 02:42 PM   #16
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Old 01-09-2020, 03:06 PM   #17
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Got it. However...

I have one more question (sorry!)

My next task was to figure out what this was, & I just did. Apparently a DV-2 automatic drain valve. Is this effective in the location where it's mounted? I'd think it wouldn't do any good unless pulling from the very bottom of the tank. According to the manual it has inputs both on the top (not used currently), and the side (currently being used). Would it make sense to just swap this with the air chuck, using the top port and plugging the side? Or leave it alone & replace the chuck w/ a petcock as planned?

Looks to be close to the bottom of the tank, which is "Good 'nuff", it gets most of the moisture out.
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Old 01-09-2020, 03:35 PM   #18
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Sweet. Thanks so much, Brad!!!! Can't thank you enough.
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