[Tim]

Hope that wasn’t on your Bus Tom!

[Archived]

I have often urged folks to establish an age at which point air bags will be replaced. The reason I stress the importance of replacing air bags before failure is because of the domino effect a failure can have.

Having the leans is annoying. Having an air bag fail catastrophically brings the issue to a whole new level.

First you can expect the coach to drop to the lowest point where the air bag failed. If it is in the rear the coach will lean substantially to one side and it will put some items on the underside very close to the ground. If it is on the front the entire front drops and you likely are going to have limited steering ability. In any case you no longer have a suspension. The bus will be banging on the bottom of the suspension.

If you do nothing your air pressure on the accessory system will drop to zero. Brake system pressure will eventually get to the pressure setting of the protection valves, which could be around 75PSI. If you have insured your protection valves work you will have pressure. If not you may lose brake pressure and at that point your brakes will lock.

Then the problem goes from bad to worse. You cannot be towed. You are going to end up on a trailer and risk thousands of dollars worth of damage to the belly of the bus.

But………..if you find yourself in this situation you can mitigate damages. The first thing is to protect against the loss of air so get the level low into the manual mode and manually raise or lower the remaining suspension locations to minimize damage, perhaps allow limited driving and to stop air from going towards the bad air bag. That will allow full system pressures, including the accessory side.

If you can drive, either get to a safe roomy place for road side service or a service facility.

Notice the failure was not where cracks in the air bag are normally seen so while inspections are going to help determine the air bag condition on your coach an inspection cannot determine all potential risks.

[Andre]

I had my air bags checked by Prevost in January. I told them, they had never been replaced, and asked if they should be. So, they inspected them, and said they looked like new, no cracks or anything. The chassis is a ’03, so the 10 year suggestion has passed. My question is, would you replace them anyway??

This is my third Vantare, and the first one I’ve owned that doesn’t have any major air leaks (knock on wood), has always been stored inside in a heated and cooled shop, tires sitting on carpet, and has never been up north on a salted road, so there is literally NO corrosion on anything. All the techs at Prevost had to come over and look underneath, because they could not believe how clean it was. The mileage is 39,000

So what would you guys do on the air bags?

[Archived]

Jerry, I was all geared up and gung ho about changing my air bags, but during the last service about 2 months ago I looked them over real well and saw no evidence they are nearing the end of their life.

They were examined with the bus at full height and then at road height and in neither position did I see any signs of cracking. I’m not changing them. I will continue to examine them at every service which is 2 or three times per year.

The bus has about 120,000 miles but I don’t know its history. It has been inside for the last 4 years.

[Peter]

I wouldn’t do anything Jerry.

[Archived]

When we bought our coach a little over a year ago and not knowing the history of the bags or most of anything else I made the decision to change all the bags last October. Prevost Ft Worth look them over and said some look like they needed changing while others didn’t, so fearing what Jon was talking about, we changed them all, to have piece of mind.

[Archived]

Just had all airbags and norgen valves replaced by jeff at ETLC..they did show some cracks at top …Jeff said I could wait and do next year..decided to do all for safety..Jeff was fantastic and his two young guys and brittany go above what is expected..Thanks to Jeff and crew as well as prevost community..

[Michael]

Thanks, I was hoping ya’ll would say leave them alone for now.

I just got back from a trip yesterday, and I had a low tag tire, so I took it off, here is a picture of the bags on left rear.

I haven’t cleaned anything under there yet.

[Archived]

Jon,

How are the front and rear air brake tanks going to go to 0? Both have check valves at the inlet and the acc tank will not allow air to enter till the supply pipe is at 90 psi. So, where does the air in the brake tanks go?

Please explain

[Archived]

I did not say the brake tanks would drop to zero, they will drop to the protection valve pressure. You will have brakes, the compressor will run 100% of the time, and you could isolate and block the leak via the level low in manual mode but with the front or one corner down it might be unsafe to drive.

But that is why we stress that anytime someone has access to the undersideof a coach, drain the tanks in sequence to verify check and protection valve operation.

[Chas]

Jon,

Please explain how the front and rear tank will go low enough to activate the parking brake. The front and rear air tanks have check valves to prevent the air from back feeding in to the supply line and the aux tank inlet device will not let air in until the supply pressure is 90 psi. It will allow air to return to the supply line until the aux pressure reaches 75 psi.

So how do the front and rear tanks get low enough to activate the parking brake?

Check the schematic and read the explaination. It is pretty well explained.

[Robert]

Mikee,

You have me confused. If you blow an air bag, and you do nothing to block off flow to that air bag the aux system goes to zero. Your brake system remains at the pressure of the protection valve. Once the brake systems reach that pressure, air passes the protection valve, but unless the flow to the burst air bag is stopped by using the level low control you cannot build accessory pressure unless the hole in the air bag is so small you retain aux or accessory system air.

If you have the older type combo brake chambers and you rupture a diaphragm that brake will lock up and if the loss of air is great enough so will the others but that has nothing to do with accessory system air. It will impact your ability to retain brake system air pressure.

I believe the Knorr Bremse use an O ring on a piston near as I can tell from a drawing and a catastrophic emergencyt brake pressure loss is less likely.

On edit, a failed hose from the relay valve will act like a ruptured emergency brake diaphragm regardless of the type of brake chambers.

[John]

Jon,

If you lose an air bag the aux pressure will go down, as far as 0. But the air compressor will start making air pressure and try to fill the aux tank, but the valve at the aux tank will not let air in until the supply line from the compressor is at 90 psi. The front and rear tanks have a check valve to prevent air from leaving the tanks to go back into the supply line. So after you apply the brakes and the tanks lose pressure the compressor will maintain 90 psi in the front and rear tanks as the aux tank will not let air in until the supply line is at 90 psi.

Yes, the coach will fall on the bad air bag corner and you would want to stop asap. But there is little to no chance the brakes will lock up due to low front and rear tank pressure.

The parking brake is not controlled by the aux tank. Both the front and rear tank can supply the brake protection valve. There is a combiner in the front run bay that handles that. As long as one tank is above the apply psi (usually 60 psi) the parking brake will not apply.

A failed brake chamber pancake is a different conversation.

The relay valve is also a different conversation.

As far as the compressor running it runs 100% of the time. Sometimes it is “loaded” (sending pressure to the system) and sometimes it is unloaded” (not making pressure) this is controlled by the air pressure governor.

There is an easy way to check the tank check valves. Drain the front or rear tank with the coach not running and the other one should hold pressure. Then do the other one.

[Chris]

Mikee, You are repeating what I am saying.

[Archived]

Jon,

You are saying the parking brake will come on and I am saying it will not . That is not the same thing.

You are also talking about a chamber failure in your last post. That has nothing to so with an air bag failure.

We are not saying the same thing.

Please explain how an air bag failure will result in the parking brake coming on. This is what you are stating.