Air Bag Bench Test: how do I do that?

[hobobimmer]

Deb and I replaced six rear air bags on our 1991 Angola. We did not bench test any, figuring Prevost would not ship one which leaked. We were wrong. Ordered a replacement for the one which was leaking and installed it. Suspect it might be leaking, too, but can't figure how we might be recipient of two faulty bags.

If we conclude it is leaking, I want to take it off and "bench test" it, but I don't know what that means or how to do. I have a "bench," so that's where I'll test it. Other than a bench, what do I need?

Exactly what do I need to accomplish this? I think I'll have to buy gauge and fittings, but don't know which ones. I live about 15 minutes from a NAPA, and we have Fastenal, Advance Auto, AutoZone, and a privately owned auto parts store. I imagine I can get what I need there. I also have big compressor with plenty of air power.

If there is an article which any of you know of which says, "Here's How To Bench Test Your Prevost Air Bag Prior To Installation So You Don't Have To Install It More Than Once," please direct me to that and I'll see if can figure it out from there.

In advance, thanks.

eric faires
huntsville, TN

[Joe Cannarozzi]

Eric

I have never done this but it seems easy enough.

I would first build a box around it. Like a 2 by 4 frame around it so when you air it up it isn't extended all the way out.

Thread a T into a nipple and into the air inlet on the top of the bag. Thread a pressure guage into one opening and a female quick coupler for your air line into the other.

Put about 40 lbs of air into it and unhook the air line from the female coupler. First do the soapy water treatment then see if it bleeds down over time with the air pressure guage.

[dreamchasers]

Finding small air leaks are always a challenge, weather on a Prevost or other pneumatic systems. Joe's idea will work, but you might consider filling a 5 gallon bucket with water and pushing the inflated air bag underwater to see if you see any air bubbles. If you have a leak, submerging the air bag under water will find the leak. I think Jon has done air bag bench test before. Perhaps he can help with what he did to confine the air bag and at what pressures.

If the air bag is still mounted, then things are a little more complicated.

Status:
The bus barn slab is poured and the driveway modifications should be completed this week. I am working on temporary electrical now.

Good Luck,

Hector

[hobobimmer]

Eric
I would first build a box around it. Like a 2 by 4 frame around it so when you air it up it isn't extended all the way out.

Couldn't I just lay it on its side? I don't mind building a box or frame for it, but wonder if I need to do that.

I'll go by NAPA tomorrow to see if they have the parts (I bet they do).

Thanks for guidance.

eric faires
huntsville, TN

[Joe Cannarozzi]

Sure you could. Watch how much pressure you put in it. Carrying a load under the bus they are around 60 or 70psi but I would not go nowhere near that bench testing without that jig.

I thought the jig would duplicate the actuall configuration better. Sometimes what is leaking at 60psi will not leak at 30.

I would be concerned what may happen at the bottom where the bag attaches to that base if allowed to just stretch out but I may be worrying about nothing.

If you have one of the old ones experiment on one of those first.

[Jon Wehrenberg]

I have a fabricated steel fixture you can use Eric. I made if back in the old days when the air bag tops popped in. I used it to pressurize the bag to seat the top and check for leaks.

With the air bag in there and a valving arrangement (that you will have to make) to seal the air bag off you can leave it pressurized and roll it around in a big tub of water. I also have the tub.

[Orren Zook]

Did you check your leveling valve(s) on that axle? It's pretty easy to see if one is exhausting when it shouldn't be and easier than swapping out a bag. Just put the exhaust hose from the leveling valve in a glass of water and see if it is making bubbles, in a level condition there should be no air escaping from the valve's exhaust port

[Jon Wehrenberg]

Orren,

I have some knowledge of where Deb and Eric are on this puzzle.

The issue is isolated to one side, rear. It was further isolated by lifting the tag axle and finding that stopped the leans. By doing that it eliminates all potential sources of leaks to a leak on the tag axle side of the 3 port valve between the tag and drives, all the fittings associated with that, and the air bag.

The new air bag was found to be leaking around the top crimp. I am assuming that air bag has been replaced with a new one and the defective new one is on its way back to Nashville for credit. If the coach continues to lean, which is sounds like is happening (assuming the tag lift exercise is repeated with the same results) it brings the 3 port valve, the fittings and the replacement air bag again into question.

If Eric borrows my air bag cage and finds the air bag to be OK my advice is to drive it and see if exercising the valves blows the gremlin out of the system. If the air bag and fittings are OK it could be as simple as a bit of dirt at the O ring on the spool valve that just needs to get exhausted out through use.

[hobobimmer]

Orren,

I have some knowledge of where Deb and Eric are on this puzzle.

Jon stated it all correctly, and has more than "some knowledge" of where we are. He has guided us every step of the way, and did a whole lot of work on our bus in addition to his supervisory role where he just wanted to watch Deb and me get greasy.

Tonight is tag axle lifting time (again), to see if can isolate the problem (again). We already replaced the three port Norgren, and this is the second bag at the tag (I can rhyme as well as Loc can pun, maybe better).

Thank you all for help.

eric faires
huntsville, TN

[Jon Wehrenberg]

What I watched was new owners really get into developing an understanding of their coach. Neither Deb nor Eric shied away from diving in to update the underside of the coach, and while there may still be some things under there for them to learn I can say they are experts now in the topics of the suspension and the brakes.

They have replaced air bags, Norgrens, brake chambers. helped with hub seals, chased leaks, and most importantly correctly analyzed where leaks could be, and did a 100% lube and fluid change in exact accordance with the Prevost specs. Very impressive.