Small but steady air leak

[ajducote]

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?

[merle&louise]

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.

[LNDYCHT]

Andre,
I'm sure you already know where to find pneumatic diagrams for your coach, if not go here http://prevostparts.volvo.com/technicalpublications/en/pneumatique.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.

[Jon Wehrenberg]

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.

[Kuhlsport]

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.

[Jon Wehrenberg]

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.

[ajducote]

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.

[truk4u]

Andre,

I can send you my electronic stethoscope if you want.

[ajducote]

Tom,

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

Thanks

[LNDYCHT]

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.

[johnklopp]

One engineers approach, Make the leak larger and easier to find by reducing molecule size. :p

If the leak is almost impossible to find, consider putting helium into the air system rather than air. The helium won’t hurt anything on the suspension; but will leak more than 10 times faster for the same size hole and pressure. Helium is readily available at most party stores for inflating children’s balloons. The reason is simply the molecule size. A helium balloon shrinks because the gas molecules within the balloon can slowly pass directly through the permeable rubber of the balloon, air does not. Tires also have a permeability ratio that will allow them to very slowly deflate. A helium molecule may be the smallest molecule in existence. A carbon dioxide molecule (Air) is much larger, composed of one carbon atom and two oxygen atoms.

Remove the helium before driving the bus.

[hobobimmer]

Here is Eric/Deb Faires suggestion. On our 92 Angola, we had a leak from the fitting at the actual gauge on the dash. Where the gauge was fastened to the air line, that's where we had a leak. Replaced the little fitting, that leak ended. Would surprise me that both gauge fittings/connections would go at same time, but might be worth a look. Was fun experience removing the dash to look behind it to see lots of wires for the first time.

Eric and Debbie Faires
Huntsville, TN

[dogear]

My bus had a leak that was taking all three air tanks down to zero in 4 hours. I had been chasing this leak for 7 months and the bus had been to three different shops and to Prevost Jacksonville twice. One shop stated correctly that a leak that large should be able to be heard but it wasn't. Once the bus was at Prevost Jacksonville I decided to change the tires and when the bus was up on the lift it stopped leaking. This gave them an initial clue as to what the problem was. They split the systems by isolating the converter side from Prevost's side and finally changed the R14 valve which fixed the issue. This leak had perplexed pretty much everyone and proved very frustrating. To Prevost's credit the second time the bus went into the shop they credited to work as "rework" and there was only a limited charge. Hopefully my experience will help someone else. Cheers

[ajducote]

Update.

Last Thursday I received the electronic stethoscope from Truk in the mail. I did not have time to use it Thursday night, and Friday I took the coach on a short trip to get a bad valve stem replaced on a tire. When I got home, I put the bus up Jon's stands and built air pressure up until the air dyer spit. Then shut the bus down. So I planned on spending Saturday morning under the bus looking for my air leak. Well, it is now Thursday and it has not lost a single pound of air since last Friday! With no leak, testing never happened. So, I am reluctant to send Truck his stethoscope back, knowing as soon as I do, the leak will reappear.

On a more serious note, what should I suspect now? I am sure the leak will reappear at some point. I am thinking a Norgren valve, maybe a spool valve did not close in the right position? Any suggestions of the most likely place to look the next time the leak appears? I really hate these types of problems.

[Jon Wehrenberg]

Andre,

I am not familiar with the Royale aux. air system but in your original post you spoke of a slow leak in both brake systems, and perhaps a larger initial leak in the aux air.

You have to mentally separate the two air systems and focus on one or the other. Yest they come from the same air compressor, but beyond where the aux air is supplied they never rejoin. Leaks do not repair themselves so if it had a leak, if will again leak, but chasing a slow leak is frustrating, and chasing a leak not currently present goes way beyond frustrating.

I suggest you anticipate the leaks showing up again, and eventually getting worse, but to relax until they are more serious. Then gather the tools, ears, soapy water, stethoscope, etc and prepare to do battle, but first study the air systems so you can take the clues the bus provides and concentrate your search efforts in areas likely to be the leak source.

Air leaks are the biggest annoyances we have relating to bus ownership and even if you do like Hector did and like I did once and got a leak free coach, it would not be long before leaks reappeared. I have moved past worrying about stuff like that until they get to be more serious.

[Charles Spera]

Hector/Jon,
My coach is not leak free but with all your help I have chased down a lot of them and have only one interesting situation to share.

Occasionally (not always), when I return to home base and lower the coach all the way I will, after a short while (couple of hours), start to listen to the aux pump cycle every hour or so. Sometimes I ignore it and it seems to cure itself in about 8 hours or so. On the other hand, Ms Barbie isn't always so patient with it. Fiddling around I have found that if I cycle the Level Low switches a couple of times with the ignition on, it will cure up immediately.
My conclusion is that I have a shuttle that doesn't always seat correctly. Since I can work around it I have decided not to go on a full bore leak and valve replacement safari for now.
See you in Austin.

[LNDYCHT]

Andre,
I would bet that when you take your coach of the stands you will find that your air leak will be there once again.

[Jon Wehrenberg]

Lee,

One thing we have learned is we have to think of air leaks in one of three ways. Brake system, aux air system, and suspension, which is actually three systems, front, left and right. A leak in the suspension will not necessarily pull down the pressure in the aux system, and conversely a leak in the aux system can go to zero and not cause the bus to lean or settle. Or the reverse could be true which is why leaks are such a big PITA.

[LNDYCHT]

The only point that I was trying to get across in my last post is the fact that system pressures and dynamics change when the weight of the coach is removed from the suspension.....

food for thought or not

[Jon Wehrenberg]

Can you elaborate? Are you suggesting supporting the chassis (or removing the chassis support) will reveal or mask system leaks?

If everything is leak free (there's a pipe dream) we have primary and secondary brake systems, auxiliary air system, and (3) suspension systems(s). I can't think of how supporting or removing chassis support will reveal or mask any leaks.

[LNDYCHT]

With the coach above ride height the ride height valves have no effect on the air bags if air tight. The air bags do not have any " back" pressure from the weight of the coach if the chassis is suspended. This situation only occurs when the coach is above ride height and supported by the chassis. The lack of back pressure will change the amount of pressure required to actuate control valves and will also mask small air leaks. If there is a small leak in the suspension system, it will only be evident when the coach is removed from the chassis support. If the air system is used to lift the coach from stands this leak will not be evident as it is masked by the volume of air that is required to lift the coach from the stands. If jacks are used to lift the coach and not the air suspension the coach will not return to the same height as there is less volume in the complete system due to the small leak.
I speak from experience as I have had this happen. My old coach would hold air when suspended but would leak when on its own suspension. Basically, I had three small leaks but changing the pressure in part of the system changed how it would leak.

[Jon Wehrenberg]

Interesting. The Norgrens which use O rings to seal are unaffected by pressure (or should be) unlike some valves which seal more effectively with increased pressure for example. I haven't experienced that because I raise my coach up to put the stands beneath it, and then raise it back up to remove the stands so I haven't relied on anything other than my air bags to lift the coach for purposes of supporting the chassis.