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Old 10-02-2014, 11:48 AM   #1
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Air system on coach

I've noticed when driving my coach, the air system seems to recharge or turn on the compressor to fill the tanks every minute and a half to two minutes. Is this time frame normal or excessive. I only noticed it because I was driving with the window open and I heard the loud air discharge (ejectors) and then soon after heard it again. I started watching the air gauges and that's when I noticed they would air back up every minute and a half or so. If I park the coach and leave it running my air does not go down. If I turn the parking brake off while in neutral while running the engine, that air does not go down either. It doesn't seem like I have an air leak because of the last two statements and I've walked around the coach and here no air leaking. When I level the coach and store it, it will hold air for at least a week and a half to two weeks which is in limitations as for Freightliner...

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Old 10-02-2014, 12:11 PM   #2
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This seems excessive to me. I hear the purge while driving, but have never seen my gage move. The parking brake pads are engaged with springs. While traveling down the road, the system uses air pressure to compress the springs. In all of the scenarios you describe for identifying there is no air leak, there's no air to the parking brake. It could be possible you have a bad seal in a parking brake release diaphragm. Find a flat spot and block the wheels. With the engine off, disengage the park brake and see if you hear a leak then.
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Old 10-02-2014, 01:53 PM   #3
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That is why I released the parking brake with the coach running, to see if I was losing air from the brakes but gauge never moves and no discharge. Only while traveling down the road, does the air compressor kick in every minute and a half to recharge the tanks.....

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Old 10-02-2014, 03:29 PM   #4
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The only other on board users that I'm aware of is the suspension and front air tap. Those should only register on the top gage (sys 2). As the air suspension is"active", it could be possible on a bumpy road our in high wind, constant adjustments would be recognized through the gauge and dryer purge.

Remember, more air is pumped going down the road due to faster pump RPM. Not sure in my mind yet how that makes a difference here, though. Unless you had a fault unloader valve.

Perhaps others who work on these systems for a living could provide you better guidance.
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Old 10-02-2014, 04:49 PM   #5
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The top gauge is the one that always goes down while traveling. The bottom gauge rarely moves unless hard in the brakes but the top one will go down then back up when the compressor comes on. It may be the roads I assume. Noticed it more while I was traveling through DC and the roads were pretty harsh! Thanks for the info though!

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Old 10-03-2014, 09:17 AM   #6
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With the parking brake released, apply the brake and have someone listen for an air leak. You may have a bad brake chamber.

You may also need to have the air drier purge valve kit replaced. A leaking purge valve will allow air to leak out and cause the drier to keep purging.
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Old 10-03-2014, 10:48 AM   #7
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If I stop the coach and sit still the gauge never moves. It only goes down while driving. I can run the coach at idle with the brake set or not set and gauge remains full and not air discharge noise. Once I am driving down the road and coach is moving around is when I notice the air filling up every minute and a half. I have checked the lines and everything else and here no leaks. I guess my real question is, when driving down the road how often does your tanks fill back up? Or does no one really pay attention. I never noticed it until I was driving with the window open and heard it ever few minutes and wasn't sure if that is normal....

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Old 10-03-2014, 08:35 PM   #8
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The compressor should not cut in if the pressure is between 100 and 130 lbs. Once.the pressure drops below 100 the compressor will cut in and bring the system back up to high pressure cutoff at 130 lbs. When this happens you should hear air vent
.... just like on a city bus

All air brake systems are the sames although the absolute pressure will vary.
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Old 10-03-2014, 08:46 PM   #9
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If it only does it while moving and you have air ride suspension. It could be from the bags dumping and refilling as you hit bumps and dips in the road. This will use air and cause the compressor to kick in. When sitting still there is no dumping and refilling.
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Old 10-03-2014, 09:25 PM   #10
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Quote:
Originally Posted by JTN8 View Post
If I stop the coach and sit still the gauge never moves. It only goes down while driving. I can run the coach at idle with the brake set or not set and gauge remains full and not air discharge noise. Once I am driving down the road and coach is moving around is when I notice the air filling up every minute and a half. I have checked the lines and everything else and here no leaks. I guess my real question is, when driving down the road how often does your tanks fill back up? Or does no one really pay attention. I never noticed it until I was driving with the window open and heard it ever few minutes and wasn't sure if that is normal....

Jason


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Don't have drawing of the compressor but one thing seems common. That is way too much cycling/leaking between no speed and operating speed. Wondering if problem is with compressor at full speed and leaking like a sieve, could shaft seals or internal valves be leaking badly with compressor at speed. Only common denominator that jumps out.
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Old 10-03-2014, 10:29 PM   #11
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If it only does it while moving and you have air ride suspension. It could be from the bags dumping and refilling as you hit bumps and dips in the road. This will use air and cause the compressor to kick in. When sitting still there is no dumping and refilling.

Sounds 100% like my symptoms..... My air usage has nothing to do with speed or anything like that, just the fact of moving or not moving which makes sense that the bags are utilizing more air while driving down the road to keep the motorhome level...


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Old 10-08-2014, 07:14 PM   #12
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Originally Posted by lefaivre View Post
The compressor should not cut in if the pressure is between 100 and 130 lbs. Once.the pressure drops below 100 the compressor will cut in and bring the system back up to high pressure cutoff at 130 lbs. When this happens you should hear air vent
.... just like on a city bus

All air brake systems are the sames although the absolute pressure will vary.
Actually it works like this: The compressor runs CONTINOUSLY, always increasing the pressure in the tanks. That is, the compressor is a DIRECT DRIVE to the engine; there is no "clutch". To keep the tanks from exploding, a device referred to as a "governor" releases EXCESS pressure every couple of minutes.

The governor "popping off" is the sound you are hearing. It will release (as needed) whether you are moving, in park, stopped at a light, stepping on the brake, NOT stepping on the brake, whatever. The ONLY thing necessary for the governor to release every two minutes or so, is that the engine be running.

Think of it as similar to your wife's pressure cooker as it sits on top of the stove. When the pressure gets to high, the safety valve will open temporarily to release SOME of the pressure, then close to start the cycle again.

I rarely look at my pressure gauges. You have two of them because you really have (at least some) brake system parts "duplexed" for safety. Just like you car has two separate (and isolated from each other) brake systems for the front and rear axles.

If your pressure ever drops below around 75 lbs you'll get an audible alert. If you choose to ignore THAT, then at around 40 lbs, the yellow knob will automatically pop out, effectively applying your parking brake for you, whether you like it or not!!!

You can test this by stopping the engine, leave the parking brake off and then (while on level ground) repeatedly "pump" the brake pedal to deplete the pressure in the tanks. At around 40 lbs, the parking brake will set itself.

FWIW, a commercial drivers license test requires that you demonstrate your knowledge of this air brake feature, as well as quite a bit of other info about the air brakes. Even though it's rarely done in "real life", part of a standard pre-trip inspection on an 18 wheeler (required by law) is a brake test that takes about 2-3 minutes to complete and checks for SEVERAL brake related functions.

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Old 10-14-2014, 09:39 AM   #13
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Pretty sure my right rear air bag is leaking now. I know it's normal when they are full and the coach is off and you walk out you here the bags release some air but my rear one is constantly making the nous of air coming out. If I rock the coach by hand I hear it more, then stops and starts again on its own.... Looks like a trip to the local FL coming hot.....

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Old 10-14-2014, 05:31 PM   #14
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Good info there BooWho. That should be a stickie, if they have stickies here.
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Old 10-14-2014, 09:01 PM   #15
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PDF of Berkshire Airbrake System

This PDF of the Berkshire Airbrake system may help the discussion
Attached Files
File Type: pdf Air Brake System.pdf (164.8 KB, 79 views)
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Old 10-15-2014, 06:43 AM   #16
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booho you are close in your explanation of the air compressor system but here is maybe a better one. Although the air compressor is direct drive it has an unloader valve in it to keep it from building pressure. When your air system reaches 130 psi or so the governor senses that pressure from the tank and opens sending air to the unloader valve and moves the unloader to stop the build up of air also at the same time it sends air to the purge valve that is in the air dryer and it is designed to open and close once to purge any excess water out of the dryer. this is the whoosh you hear when your system is at full air pressure. When either of your gauges drop to around 100 psi or so the governor moves back exhausting the air pressure on the unloader valve starting the process all over. If you are hearing the whoosh of air repeatedly without a drop in air pressure you have a problem in this system that could cause premature wear on your air compressor.
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Old 10-16-2014, 05:51 PM   #17
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booho you are close in your explanation of the air compressor system but here is maybe a better one. Although the air compressor is direct drive it has an unloader valve in it to keep it from building pressure. When your air system reaches 130 psi or so the governor senses that pressure from the tank and opens sending air to the unloader valve and moves the unloader to stop the build up of air also at the same time it sends air to the purge valve that is in the air dryer and it is designed to open and close once to purge any excess water out of the dryer. this is the whoosh you hear when your system is at full air pressure. When either of your gauges drop to around 100 psi or so the governor moves back exhausting the air pressure on the unloader valve starting the process all over. If you are hearing the whoosh of air repeatedly without a drop in air pressure you have a problem in this system that could cause premature wear on your air compressor.
Thank you for the additional (and even more detailed than mine) explanation. I suspect you drove a lot longer than I, or that you worked for an AB manufacturer (or some such). Cajun, glad I could get at least partly correct info out.



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Old 10-16-2014, 05:55 PM   #18
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This PDF of the Berkshire Airbrake system may help the discussion
Outstanding document. Thanks for sharing.

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Old 10-16-2014, 07:53 PM   #19
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30 years bending wrenches on class 8 and heavy construction equipment cant sit in the seat long enough to be a driver
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Old 10-21-2014, 11:33 AM   #20
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Took the coach to a Freightliner service shop this morning and they found the leveling valve to be bad. Part should be here in a day or two. My coach just went past a year old but looking into the Freightliner warranty, looks like it is a 3 year/50,000 mile warranty so I should be covered.
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