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Leaking service air brakes


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Just came back from a very nice weekend of camping. On the way back into town I noticed that both air tanks were cycling between 125lbs and 90lbs fairly frequently, maybe every 10 mins or so, which is not normal. Then when I was making the short trip to the storage lot I started losing pressure in both tanks quickly, below 90lbs. I was just able to roll into my spot before the emergency brake engaged due to low air pressure.  Not the kind of excitement of I was looking for.

Things I noticed while driving:

- Large hissing sound from under the middle/rear of the cab section of the coach.

- Small hissing sound from under the brake pedal itself when holding the brakes in (normally no hissing while holding the pedal at a stop).

Now that it is parked, I am unable to build more than about 30lbs of pressure in both tanks at fast idle (1200 rpm).

Got under the coach, sprayed around the front tank area with soapy water and quickly found the leak, a lot of air coming out of the purge valve on what I *think* might be the air brake ABS modulator. I'm basing this on the Monaco Dynasty air system chart I have (general location) and the fact that it has:

- 2 x lines, one each running to each of the front brake chambers.

- 1 x smaller line from the top of the front tank, where the diagram says the brake pedal connects to the ABS.

- 1 x feed line from the secondary/front air tank

- 1 x another line, or maybe it's an electrical connector, not sure.

Too complicated to be much else, no?

According to the air system diagram I have this could be Monaco part #01807254.

Can anyone confirm that I'm dealing with a faulty ABS Modulator?

Is there any chance the problem is actually elsewhere, like the brake pedal unit itself?   I did hear light hissing from the pedal while stopped but am wondering if it would do that if the pressure was being bled off by a faulty purge valve on the ABS unit?

Also, any idea what make, model, part# I should be looking for for a replacement? I didn't see any writing on mine in the pictures I took (this is for an 1997 Monaco Dynasty, 36', single axle).

LeakingDumpValve.jpg

Bottom.JPG

LeftSide.JPG

RightSide.JPG

TopAngle.JPG

FrontTank.png

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Just curious if you resolved this. If it's leaking out that bad have someone release the parking brake while you walk to the back of the coach. You may have a bad maxi/parking break chamber / diaphragm. ( so many different names). 

The air in the cab sounds more like the air charging or discharging through the system. If you have a bad parking break chamber leak you will hear moving air that sounds like a leak. It's just a lot of air moving around. 

Check both with the break set and stepping/ holding down on the break pedal. Listen for leaks. And then breaks released wheels chocked maybe. Engine of will help a lot.

Remember breaks will set automatically at or around 60psi. So if they ever get low..look out it will lock up!

 

20220514_031059.jpg

Edited by Romeo84
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6 hours ago, Romeo84 said:

 

Remember breaks will set automatically at or around 60psi. So if they ever get low..look out it will lock up!

 

20220514_031059.jpg

Lower than that. Pretty sure it's in the 35 to 40psi range.

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Posted (edited)

Yes, 30 PSI is when mine engages. So the leak occurs with or without the parking brake engaged. The leak is in the front brake system and the front brakes don't have spring/parking brakes, so it makes sense that having the parking brakes engaged or not makes no difference.

It seems like the component I have circled above is failing and simply dumping all the air that goes into it from the tank. But I am wondering if the brake pedal treadle could be the cause, if it was resulting in continuous air pressure being fed to the abs modulator, resulting in the modulator dumping air.  But logically, as far as I understand how this system works, the abs modulator should only dump air if the air pressure from the pedal is less than that of the front brakes (when the brake pedal is released after engagement). In my scenario, there is no pressure in the front brakes (brakes are not applied), and the air is continuously being dumped.

I will need to remove the leaking valve and clean it up to see if I can figure out what brand or model # it might be. If anyone has any idea which company might have provided this part on a '97 Dynasty I'd be happy to hear it.

 

 

Edited by RoadTripper2084
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The valve you circled in the diagram is screwed directly to the primary air tank.

I 'think' the part you have sprayed down in your pic is the air dump valve for your front brake chambers. That's where the air is released when you take your foot off the brake treadle. It should close back up as soon as the pressure is released from the two brake chambers.

Follow two of the lines off that valve and see if they go to the brake chambers.

Edited by 96 EVO
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Posted (edited)
12 minutes ago, 96 EVO said:

The valve you circled in the diagram is screwed directly to the primary air tank.

I 'think' the part you have sprayed down in your pic is the air dump valve for your front brake chambers. That's where the air is released when you take your foot off the brake treadle. It should close back up as soon as the pressure is released from the two brake chambers.

Follow two of the lines off that valve and see if they go to the brake chambers.

You are correct, this was my original thought as well. The top smaller line goes from the small valve assembly off of the front tank to the valve, this seems to be the control line from the pedal. The two lines on each side go directly to the brake chambers on each side. There is also the larger source air line from the front tank to the lower center of the unit, and then one more larger line above that and to the right. Not sure where that one comes from.

Upon further reading it seemed like this was the ABS Modulator unit, perhaps combined with the function of a normal brake relay valve.

 

Edited by RoadTripper2084
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You may be correct. Looking at this diagram, it looks like the middle valve that is leaking may be the quick release valve for the front brakes. And that the hoses go from it to the ABS modulator units on each brake can. 

629527816_ScreenShot2022-06-14at1_12_35PM.thumb.png.62a4ef2b3cca51f02e16ac2687193818.png

I had found another diagram earlier that made it seem like it could be the ABS modulator, which I can't seem to locate now.

Any ideas on what the brand or model might be?

 

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Looking at your pic and seeing what appears to be an electricaly switched valve, I would compare it to ABS relay valve. Possibly used for ATC if you have that along with the individual wheel modulators? This one is a new type from Wabco or Meritor.

Screenshot_20220614-170651_Chrome.jpg

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18 minutes ago, Ivan K said:

Looking at your pic and seeing what appears to be an electricaly switched valve, I would compare it to ABS relay valve. Possibly used for ATC if you have that along with the individual wheel modulators? This one is a new type from Wabco or Meritor.

Screenshot_20220614-170651_Chrome.jpg

That does look very similar to mine, I guess the thinner top line could be electrical as well. 

I don't believe I have ATC but couldn't say for sure. It was one of the earlier ABS equipped coaches, back when they still put "ABS" on the back of the coach as a feature badge. 🙂

Can I ask where you found that picture?

 

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Certainly,  I just googled ABS relay valve and got hits like this: https://www.wabco-customercentre.com/catalog/en/4721950310

Edit: I see that this one link happens to be from UK but at least it gives a part number if it is a physical match. I am curious, if you don't have ATC as I don't either, do you have individual modulator valves or have they used this one instead? I believe these are meant for trailers but I could imagine them on the front as well in the early days.

Edited by Ivan K
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Posted (edited)

So that certainly is a close match for mine in terms of overall shape and number of ports, etc. Of course I need to remove mine to see if I can find a part # on it to be sure. I have no idea about the actual specs, etc.

I did find this model that was 12v instead of 24v:  https://www.wabco-customercentre.com/catalog/en_US/4721950340

The explanation of how it functions also matches my own expectations:

1987221669_ScreenShot2022-06-14at5_09_08PM.png.ac937705192f7db36d5a2adbcdc1972a.png

 

Thanks for this!

Edited by RoadTripper2084
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Posted (edited)

So another thing that is bothering my about this leak is that if the leak is in the front service brake abs relay, shouldn't that be solely connected to the Secondary/Front air tank?   With this leak I'm seeing both tanks deplete, the primary down to approx. 25psi and the secondary to zero. 

I am also unable to add enough air at fast idle to gain more than approx. 30psi in both tanks.

I do see a line connecting the secondary / front dry tank to the rear primary tank, which also connects to the same outlet that the front service brakes do, via the abs relay which is leaking out the exhaust port (see below).  I'm confused as to why/how the primary tank connects to the secondary tank in this manner. It would seem to violate the isolation of each tank circuit. 

FrontTank2.thumb.png.f41794b298083dc90d757c859ce8d8ff.png

RearTank.thumb.png.33edb574f5ed8155d88cbbd557c45e23.png

As you can see in the diagram, the wet tank (half of the front tank) connects to the rear tank via the Red 5/8" line that has a check valve to prevent backflow into the wet tank half of the front tank. But there is also the Silver 1/2" line that seem to directly connect the rear tank to the front dry tank at the service brake fitting, with no apparent check valves. This would explain why both tanks are leaking out of the front service brake abs relay, but doesn't really seem like a sound air brake design, I would have expected the rear tank to be reserved for the rear service/parking brakes.

 

Edited by RoadTripper2084
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I have the same charts for our 2000 showing direct connection between the dry tanks, however, why would you have some pressure in one and not the other. Granted, the pressure readings come from the brake pedal assembly which might still hold some internally eventhough the tanks are empty? Weird. Mine seem to leak down at the same rate without the compressor running and I thought it made sense.

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For the service brakes, they need to be separated into two independent systems. 

Have a massive air leak on the front axle, the rear axle braking needs to continue to work. 

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36 minutes ago, 96 EVO said:

For the service brakes, they need to be separated into two independent systems. 

Have a massive air leak on the front axle, the rear axle braking needs to continue to work. 

But why would there even be a line between the dry tanks then? How's your chart?

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My manual didn't include a air diagram. 

Harry sent me a bunch of files for our coaches. I'll have to look through them and see if an air schematic is in there.

Ok, found it in my air brake course manual. That valve Ken has circled is called a 'two-way check valve'. It will supply air from either the primary, or secondary dry tank (whichever has the most pressure), to keep the parking brake dash control switch from applying the parking brake unnecessarily!

Notice one line from it goes to spring relay, the other side parking brake switch.

Edited by 96 EVO
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14 hours ago, 96 EVO said:

My manual didn't include a air diagram. 

Harry sent me a bunch of files for our coaches. I'll have to look through them and see if an air schematic is in there.

Ok, found it in my air brake course manual. That valve Ken has circled is called a 'two-way check valve'. It will supply air from either the primary, or secondary dry tank (whichever has the most pressure), to keep the parking brake dash control switch from applying the parking brake unnecessarily!

Notice one line from it goes to spring relay, the other side parking brake switch.

Okay I found another diagram from a difference source that does show there is a 2-way check valve in the mix.

255756714_ScreenShot2022-06-16at8_34_08AM.thumb.png.de14a91383ce160d8f5ee21654b78889.png

So the purpose of the 2 way check valve is to provide air to the abs relay for the front service brakes from either the front (secondary) air tank or the rear (primary) air tank, whichever has the highest pressure wins.  Idea being that if one air system fails and loses pressure the other can keep your brakes working.

In my case though the front abs relay is stuck dumping air - that just means that each tank takes turns dumping all its air out the relay trying to keep the front brakes pressurized. The reason the rear tank wasn't complely empty (25lbs or so) is probably because the tolerance of the 2-way valve requires higher pressure than that to switch inputs.

So my abs relay is either failing in such a way that it:

A) Thinks it should be quick releasing the front brake air continuously. In order for that to happen "normally" I think the air pressure from the brake pedal would need to first be present (brake activated) and then release. Since I have little air pressure and am not working the brake pedal (though I did try pumping it a few times, no difference), that seems unlikely.

B) Is just fundamentally borked in such a way that the input air is being vented directly out the vent. 

Sounds like B is most likely. Okay, plan is to pull the abs relay tomorrow (if there isn't 6" of water pooled under the rig after all the rain we just had) and then tear it apart to see what I can see and look for a part number.

 

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Yeah, that two way check valve you have circled has nothing to do with service brakes. It keeps your parking brake from automatically coming on if you still have air in either front or back systems.

They don't want you being a 'sitting duck' in the middle of the interstate, when you still have an operational brake system to get you safely off the road.

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2 hours ago, 96 EVO said:

Yeah, that two way check valve you have circled has nothing to do with service brakes. It keeps your parking brake from automatically coming on if you still have air in either front or back systems.

They don't want you being a 'sitting duck' in the middle of the interstate, when you still have an operational brake system to get you safely off the road.

This make sense and the 2-way valve was not mentioned in the old charts. Looking through wheel well,  it appears to be this one, equivalent to Wabco #4106.

Screenshot_20220616-120541_Chrome.jpg.7d530c1b9f98d8e99d1da49d40180e01.jpg

Part no. KN25080 Haldex

Shuttle Type Two-Way Check Valve

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