Air Bag Bench Test: how do I do that?

[Jon Wehrenberg]

A footnote to all of Eric and Deb's problems. The original problem appears to be two leaky brand new air bags in a row. The old, presumably leak free air bag now installed will prove that as soon as this new problem can be corrected.

Just so everybody has the mental picture required, when the key (on almost all of our coaches) is "off" the air in all air bags should be locked in place by a five port valve that goes to its normally closed position. When the key is on, if the leveling switch is set to the road or driving position the path through that five port valve to the air bags goes through the ride height valve. In Eric and Debs case they have the bus fully raised so it is easier to work under the bus, and that ride height valve arm is in the position to exhaust air. Ordinarily it would not exhaust air with the key off, but in this case it appears the five port valve has decided to not fully close and is letting air pass through to the ride height valve which is exhausting air trying to get the bus to go down to the normal ride height. If the support stands were removed I suspect the bus would drop on that side until it reaches the normal ride height, at which point the ride height valve will stop exhausing air.

I know how much of a PITA this problem is by having worked for weeks to make my previous coach leak free. I also know how obsessive an owner can be when they realize how close they are to perfection and want to make that last little leak go away.

Here's the really scary part in all of this. Imagine if you will that this was happening on your bus. And unlike Deb and Eric you had no choice but to bring your bus to Prevost for air bag replacement. A slow leak in a new air bag would not show up as the leans for 12 to 24 hours. By then the bus is home leaning over in your driveway. So back you go, and if the techs do not fully understand the system some serious dollars will be spent by you until you either give up, or the problem is isolated and repaired. Doesn't that give you a warm toasty feeling?

[JIM CHALOUPKA]

Eric, if I am correct you are now testing your system relying on the fact that a previously good air bag when re installed will be leak free.
There is however the possibility that in the removal, and re install process that something happened to that bag and it will leak. Of course you have already gone through the process, so you might as well follow through. If the bus stays put you succeeded.

I think a better way to test the original air bag in a speedier fashion (less time waiting for parts, and less remove and replace), could have been the instalation of a cap on the end of the line feeding the suspect airbag, thus isolating it from the system and proving that anything beyond the cap is at fault for whatever reason.

[Jon Wehrenberg]

Jim,

That has been done, albeit in a different fashion.

By using the tag axle lift feature the drive axle air bags are isolated from the tag axle air bags via the three port Norgren.

Eric and Deb have done that and thus can rule out anything forward of the three port Norgren as being a potential leak. At least up until the apparent problem with the five port Norgren.

In both instances Deb found leaks at the top crimp on the tag axle air bags. She is incredibly fortunate in that the leaks showed up where she could see them blowing bubbles. The odds of that on an installed air bag are 50/50 because unless you start pulling wheels or fender liners at best you can only see 1/2 of the air bag.

Although you may not be aware Hector is going to provide a design for a bubbler device as soon as he gets his barn building project under control. Using Hector's device will allow exactly what you describe by making the bubbler in essence the cap and being able to see if anything downstream from the bubbler has a leak even as minute as a bubble. There are 5 or 6 different type fittings on all our suspension, so with the bubbler and five or six adapters, starting at the origin, a person could methodically work through an entire coach testing each and every component until the entire system is bubble free.

[JIM CHALOUPKA]

Patiently waiting on the sidelines for the outcome

[hobobimmer]

Here is the latest. NO LEANS after tag bag was replaced by 3rd new one from Prevost. This means that 2 out of 8 they sent to us had leaks. 25%. We will be testing any and all bags in the future prior to installation. Related to that, using a metal jig that was only safe to pressure up to 40 psi, both defective bags (which produced a very characteristic bubble pattern when installed on the bus) showed NO signs of bubbles under water. This means, that they must be tested under higher pressure than 40psi, and we don't know what that magic number is because we have to ship the bad bags back to prevost before we leave for our trip next week. We will work on a suitable design for a metal jig that can be used to test bags off the bus, but we are not experts in this area. If someone has some ideas, please share.

Now, to update the big strange 2nd noise we heard Sunday, in front of the mud flap on the drivers side, just in front of drive tire... We believe it to be ride height valve exhaust tube. At Jon's suggestion, we "exercised" the level low/drive valve by switching it back and forth numerous times and adding and removing air. Each time we did this, leak got smaller and smaller, until it finally stopped. So, I assumed that leak there was indeed related to some crud that must have gotten stuck in the 5 port. Then, it came back after the next test drive when we put it in level low, but not to the degree of the first time. For some unexplained reason, it seems to stop if you raise and lower the tag axle. I don't know what relationship there is between these. I really didn't think they were related. It may just be coincidence, but 2 times this has worked now. I'm going to call Prevost service to see if they have any ideas. Anyone out there want to guess?

But for now, we are feeling better. I have learned so much from this POG forum and from Jon and Hector. Thank you all!!!!!

BTW, Prevost parts man told me Sunday when we ordered our 3rd new bag for the tag, that he had "never heard of a leak happening from the place where I was describing." I assured him I had seen it with bubbles and heard it with my ears and it was leaking. He did not sound like he believed me. It was unbelievable to me we could have gotten 2 bad bags in a row, but now I am proven right. Yes, it does feel good! I think I have mastered the bubble technique in the process, and I am highly skilled with my ultrasonic leak detector and electronic stethoscope as well. I'll happily pass my new knowledge to anyone in need.

[Jon Wehrenberg]

What Eric and Deb did not point out is that as a result of the air bag leaks they got real good at removing and replacing the tag axle air bag on their coach.

I was told the replacement of the third one took 24 minutes.

That is impressive.

[Joe Cannarozzi]

Well this one is really distressing.

When Deb and Eric found the new bag leaking (twice) it was at that crimp where the bag meets that top plate, a mechanical failure of sorts.

The one I have been handed has small pinholes all over it, and it was HELL to even find them. Initially I literally triple checked them after instalation and got up this morning and the left rear was COMPLETLY deflated. So under I went again going through 2 bottles of the soapy goo and found NOTHING. Even put in another 3 port norgren, it was the last original component back there. Went out in the darkness to check again after 5 hrs. and nuts the thing is going down again. Everything has been replaced 5 and 3 port norgren, ride height valve and 3 new bags.

Up until now when checking new bags I would spray that top ring and around the bottom bell. Well this piece of crap is perforated ALL over it. I think If I did not go and look in the darkness with a flashlight I would not have seen it.

If I got up in the morn and found it down again I had it in mind to rig up a line so I could hook to each bag one at a time and check them independently.

Looks like I am now gonna make up a gadget to do just that and now these darn things are ALL gonna have to be bench tested.

Beautiful, many have decided to spend the extra time and money and pull everything when doing this job and it looks like Zimbabwe could produce better parts.

Well Mr. Goodyear thank you for the great quality control.

[Jon Wehrenberg]

Since all air bags have the same size pipe threads a T fitting threaded into the bag, with a Schrader valve and very sensitive pressure gauge on the other two legs of the T would be all that is needed.

However............I am not convinced a low pressure test will reveal all the leaks.

Many moons ago when the air bags had tops that snapped in I built a cage so they could be inflated to snap in the tops. I still have the cage, and if it were properly reinforced it could be used to contain the air bag while the air bag were inflated to the maximum pressure (around 100 to 110 PSI).

But that would involve 8 days of testing (one bag per day) to find the most minute leaks on all 8 air bags required for a coach.

Or, and here is a wild idea, maybe, just maybe Prevost could be expected to improve their QC or get a different supplier who can make air bags that do not leak.

[Joe Cannarozzi]

What a clever idea, the one about getting better parts.

I don't think it would take a whole day to see. Heck as soon as you see any loss in pressure at all your done, its JUNK.

Maybe Mr. Goodyear could provide us with the test cages since they now seem to be necessary. Someone could get seriously hurt testing with a home made cage that wasn't strong enough.

I'm thinking basic shop press could be used with some simple additions.

[hobobimmer]

What a clever idea, the one about getting better parts.

Is there another manufacturer of identical, but possibly leak free, bag? I think I have heard that Firestone makes bags. Anyone else? Would they possibly be direct replacements?

Joe, we feel your pain dealing with leaky air bags that should not be leaking.

eric and debbie faires
huntsville, TN