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Thread: Small but steady air leak

  1. #1
    Join Date
    Apr 2008
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    no where
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    Default Small but steady air leak

    Well after 6 years and no air leaks to speak of, I have my 1st air leak that I can not find.

    Symptoms:
    1. Key off
    2. Level low in drive position
    3. parked on a level spot
    4. Air bags holding air, NO LEANS
    5. Primary and secondary air gauges lose about 1.5 lbs an hour.
    6. They will eventually go down to zero over a period of 3 days or so.
    7. They go down in sync. Both primary and secondary seem to lose air at the same rate.
    8. Aux Air guage goes down to about 70 Lbs and holds there.

    I have sprayed soapy water all over the air tanks, their fittings and drain valves with no luck. I have to believe that since both tanks are going down at the same rate that there must be something in common with both that I need to check.

    I am thinking that it could be:
    1. Air compressor ?
    2. Air governor which I have been unable to find...??
    3. Air dryer?

    Any others I should look at? And how do I test the above 3 components?
    It is a very small but persistent leak and I may have to wait until it gets bigger to find it, but I don't like that idea. What makes me frustrated about this leak, is that until now the primary and secondary air gauges NEVER lost so much as a pound over weeks or months. Those gauges were rock solid at whatever pressure was on the system when I shut down the bus.

    Any ideas where to look?

  2. #2
    Join Date
    Aug 2006
    Location
    Houma, LA
    Posts
    1,783

    Default

    Andre,

    I think I can help you find the air governor. Locate the starter and the starter solenoid on the starboard side of your engine. There should be an 1/4" air hose close by. Follow the air hose traveling toward the back of the coach and you will find the air governor.

    My air governor crapped out while I was dry camping. Next morning when I started the engine it would not go in gear. I checked around the air governor area and sure enough I could feel an air leak.
    Tuga & Karen Gaidry

    2012 Honda Pilot

  3. #3
    Join Date
    Dec 2010
    Location
    Penetang
    Posts
    117

    Default

    Andre,
    I'm sure you already know where to find pneumatic diagrams for your coach, if not go here http://prevostparts.volvo.com/techni...neumatique.asp
    The placement of the valves and check valves should help you isolate the leak. Air valves are clearly indicated and check valves are identified by an arrow. I would doubt that it is the compressor unless the check valve has failed.

  4. #4
    Join Date
    Jan 2006
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    anytown
    Posts
    8,908

    Default

    Andre,

    Following up on Lee's points You gave some significant clues. Your leak relates to the braking system only or primarily. If the primary and secondary gauges are dropping the leak is somewhere between the compressor and the valves that actuate the brakes.

    You can rule out anything relating to the brake chambers because when parked there is no air to them. Air to the chambers stops at the relay valves. So by isolating your quest to those devices from the compressor through the entire system up to the relay valves you are likely to find the leak. Lee is correct that the pneumatic diagram will help you with where to look.

    FWIW 1.5 PSI per hour is a very small leak. The fact that both systems leak down the same rate look for points common to the secondary and primary. Brake treadle? Air dryer? Check valves, especially at the compressor? In a perfect system you should be able to drain air from each air tank in order and still have pressure on the next one.

  5. #5
    Join Date
    Jan 2011
    Location
    Cincinnati, OH
    Posts
    23

    Default

    I have this same problem on my new coach. I logged on to do some searching for answers. Mine is a slow leak and both primary and secondary go down at the same rate. Thanks for the help.

  6. #6
    Join Date
    Jan 2006
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    anytown
    Posts
    8,908

    Default

    For anyone who has not yet chased air leaks, or who is new to the Prevost attempting to find every leak and repair them all becomes a frustrating obsession. I have no idea how many air fittings we have, how many potential leaks we have, or how to stop the aging process on the rubber components used to prevent leaks.

    What can be said is if you have leaks and you turn your coach over to a service center or mechanic to fix them it is going to get expensive. I am not suggesting you ignore leaks, but like Andre, you provide detailed information about the characteristics of the leaks to help a knowledgible person to isolate the area to be investigated. If the mechanic understands the coach, and the information provided is accurate the effort required is minimized. Those of us who have chased leaks know there is no single best way, or easy way to find them. The big ones are easy because you can hear them. The smaller leaks, especially those in inaccessible places are much more difficult to find. So far I know of six ways to detect leaks. We can listen for them. We can use a soapy solution. We can use a stethoscope. We can use an ultrasonic leak detector. We can use Freon to pressurize the system and then use a Freon leak detector (use caution here). And finally we can use a device Hector will show during a seminar in Austin in October that allows bubbles in water to signal a leak in a circuit.

    Each method has its positive and negative aspects, and for the determined owner the use of them will eventually identify every leak. If the owner turns the responsibility over to someone else there is a financial aspect to leak detection and some owners have spent thousands chasing perfection.

  7. #7
    Join Date
    Apr 2008
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    no where
    Posts
    439

    Default

    An update. Thanks for all the input, it all helps.

    I have been studying the Prevost diagram and have started a very slow and methodical check of my air system. The leak is so small, I not sure I will find it, but I want to try.

    I started back at the air compressor, spraying soapy water and checking for loose fittings. So far no bubbles, but I found 1 small fitting on the "unloader" on the back of the air compressor that was just a 1/8 turn loose. Also the 2 bolts that hold the "unloader" on the compressor where just a tiny bit loose. So I tightened them up. It does not appear that is where the leak is. SO next stop is the wet tank fittings and all the fittings on the air dryer.

    I have the bus up on Jon's stands and I am able to safely crawl under the bus and check 1 or 2 parts at a time. Since I have found a few very slightly loose fittings, I guess I will put a wrench on every fitting I can find and see if I stumble onto the leak.

    Just to be clear, it is a VERY small, Very slow leak. I could ignore it for now and wait to it gets a little bigger and it might be easier to find. BUT, until a week ago the air system on this bus has been as tight and leak free as they come. It was so good for so long, I really want to get it back to air tight if possible.

  8. #8
    Join Date
    Jan 2006
    Location
    Polk City
    Posts
    3,676

    Default

    Andre,

    I can send you my electronic stethoscope if you want.
    Tom

  9. #9
    Join Date
    Apr 2008
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    no where
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    Tom,

    Thanks for the offer, that would be a big help. Will send my address in email.

    Thanks

  10. #10
    Join Date
    Dec 2010
    Location
    Penetang
    Posts
    117

    Default

    In my experience chasing leaks it is seldom fittings and usually devices such as valves. Both of my coaches had leaks at the belt tensioner control valve when I purchased them. The new coach also had a leak at tag axle valve, this leak was very small and not always present.
    I found the best method was a stethoscope which is easily obtained at a tool store, I extended the hose on mine with a piece of tube and removed the amplifier device from the end so that it is strictly hose. You definitely know when you are close to a leak.

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