Air Bag Bench Test: how do I do that?

[lewpopp]

Hey Eric, can you tell us any good jokes? Oh, I forgot, this forum is not censored.

[Joe Cannarozzi]

So Eric, what happened with that bag and that leak. You cannot just leave us hanging.

What about that bench test?

So now to be completly sure one has to pretest bran new components? Sheesh. What kinda crap is that? Thank you Mr. Firestone.

[Jon Wehrenberg]

I think the airbags were BFGoodrich, but you are right. They shouldn't leak.

Eric and Deb picked up the airbag cage and a big tub last night. If the second new airbag is found to leak this brings a whole new dimension to the difficulties in tracing a cause for the leans. It is relatively easy to find a leak in a fitting, less easy to isolate a leak in a Norgren valve, and very difficult to find a leak in an airbag.

For those who are not aware, unless an airbag leak can be detected with soapy water (remember, unless you pull the wheels and front drive axle fender liner only 1/2 the airbag is visible, and none is visible at the bottom) there is no other way to find an airbag leak while it is installed.

That means the best and only way is to remove the airbag and inflate it, and put it in a tub of water and look for bubbles. What a pain in the butt that is. An ultrasonic leak detector will not "hear" and airbag leak.

[phorner]

Does it make sense to make it a part of the installation procedure to test all new bags prior to installation, or is this a pretty rare occurence?

Seems like it would be a real pain to have to test every new air bag, but I guess it would give some sense of insurance that you don't have to do it twice (or more) times to achieve a leak-free installation.

As I have 6 replacements to go, I'm sure hoping that this is a very rare problem

[Joe Cannarozzi]

I remember seeing a photo of a metal jig for inflating bags out of the bus. It was in the prevo factory service manual we got with ours. That I believe was designed for when the bags were different and the tops came separate as Jon mentioned.

In any event if you had such a jig and had new bags I do not think it would be too much extra time to air them up before putting them in.

Good Grief.

[Jon Wehrenberg]

Paul,

Over the years I have installed 16 air bags on my buses and helped with a smilar number of installs. I was replacing them when the tops were separate and had to be seated using a cage which I built.

Until Eric and Deb found the leak in one of their new airbags I have not had a problem. The current air bag designs have done away with the removeable top and have a metal top that is rolled and crimped over the rubber. That is where Deb found the leak in a new air bag.

Since they are having the leans with the second new airbag, and they have replaced the 3 port Norgren, and isolated the problem to the new Norgren, the new tag axle air bag or the associated fittings it points to the air bag again. So to answer your question I think from now on I am planning on testing air bags before they are installed because it is time consuming to remove and replace them after they are installed, but more importantly, if they do leak you spend a lot of time ruling everything else out.

I used to bench test Norgrens before I installed them using a rig I made up of Home Depot valves and fittings and hoses. I am going to go back to doing that also.

Besides Roger, Deb and Eric, I know Brian and Jim Shoen have replaced air bags recently. I don't know if they have had any problems. This may just be a fluke.

[hobobimmer]

So Eric, what happened with that bag and that leak. You cannot just leave us hanging.

What about that bench test?

Sorry, Joe. Did not mean to leave you hanging. Bench test results. After picked up Jon Wehrenberg's steel cage last night, tried it this morning. Used40 PSI in bag. Put under water in our utility sink. Could not get bubbles.

Jon used this "cage" to seat the bead on old style air bags. It was not built (or, in the ordianary Jon W. manner of operating, overbuilt) to handle the full pressures of an air bag. So in conversation this morning, he advised me not to inflate more than the 40 PSI without some additional structural steel in the cage to insure it could handle more PSI within the air bag. Before I ship the bags back to Prevost under warranty (since they've not seen one mile of service), we're going to strengthen Jon's steel cage so we can test at 100 PSI. We want to reproduce the leak we know is there with a bench test. Then we'll employ same method to test BAG NO. 3 before we intall it.

But with air bag number 2 of the new ones at the left rear tag (Deb wants to know if anyone feels sorry for us yet or if we're just preaching to the choir), and with Deb re-checking left tag bag for leaks again today and confirming leaks, we opted to swap out BAG NO. 2 for what is now the third bag in that location in the last month or two. I used one of the older bags which we removed when we put the six new ones in. Hope it holds. We'll buy another, but will bench test before installation. By the way, tooting my own horn regarding my install time, I did complete removal to completed install of the left tag bag between 1:42pm and 2:07pm (25 minutes). I know there is not another bag on the bus I could do in that short time, but this bag I did.

So we aired it up to see if we'd get the leans in about 24 hours like we have been. Deb gets under bus with her ultrasonic leak detector, and I'm sitting between tag and drive on outside of bus. We both hear huge air sound, different from any we've had to date. Traced sound to in front of drive wheel mud flap, and we think it's called ride height valve exhaust port (at least that's what I think it is until Jon tells me it's something else). It caused the leans in about 10 minutes.

So call to Jon about new leak. We "exercised" our valve, at his suggestion. Did the Level Low up, down, left, right, front, drive, over and over again. And then over and over again, because the leak was still there. And then over and over again, because the leak was still there. And then over and over again, because the leak was still there. And then it returned but smaller. And then it seemed to quit. We'll see in morning. Maybe the air bag will hold this time, but the ride height valve will be the culprit. Fingers crossed that both hold.

So that's the update.

eric and debbie faires
huntsville, TN

[Joe Cannarozzi]

Thanks Eric.

No one feels sorry for you. We know what you are up to.

You need not put that much in those bags to bench test them. I am going to go out on a limb and say I would be suprised if you had to even get to 75 to get it to start leaking again. They will never see 100psi during normal operation.

That ride height valve will "leak" actually exhaust air by design and it sometimes takes a very long time to bleed off when it needs to, gradually tapering off until it stops.

I am confident you are aware of that and the leak there is more than I have described.

I was thinking about installing a suspension pressure gauge but not to monitor normal operation. I thought it would be good when in level-low to tell me when I had topped out at any one corner and would thus prevent unnecessarily overinflating the bag by staying on the trigger after the suspension runs out of travel. We are not auto-level but have the old manually opperated system.

Trucks often have suspension air gauges. An axle carrying 17000lb has around 60 to 65psi in the bags. With our drives on the bus at 20000lb it will be about 75psi

[phorner]

Eric,

I give you and Deb a LOT of credit for tackling this project. It must be extremely frustrating to do everting possible to do the job right (which I'm sure you are, given the work ethic of your mentor) and still have to repeat the performance.

Although it was no where close to the work that you are undertaking, I can understand to a small degree what you're going through. I once had to replace 3 BRAND NEW thermostats that I was installing because it was the "right thing to do" while replacing radiator hoses on one of my cars. As hard as it was for me to believe, 2 out of the 3 new thermostats were defective. It was fun taking things apart the first time, but by round 3, well, my vocabulary had changed a bit.

Good luck in getting this job completed once and for all. I'm rootin' for ya !

[hobobimmer]

Eric,
Good luck in getting this job completed once and for all. I'm rootin' for ya !

Even though Joe doesn't feel sorry for us, we're glad you're rootin' for us.

As of 6:10am today, it was leaning a bit but not much. Ride height exhaust was leaking, but slightly.

Deb will check my fittings on tag air bag install later this morning, and we'll take bus for a ride to see if that form of "exercise" improves our leans by perhaps blowing crud out of or through system. Fingers crossed.

Thanks, all.

eric faires
huntsville, TN