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Engine compartment air bags deflate quickly
When I bought the coach I noticed that the air bags for the fan gear and the OTR compressor deflate overnight and leave the belts very loose. When I mentioned this to the seller, he said they all do that. I doubt that. Now it could be a “feature” to remove belt tension on bearings for prolonged storage. Do your fan gear/OTR compressor belts lose tension? How quickly? The rest of the bus seems to hold air well at least at ride height.
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22 Replies
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The engine bellows get air from the aux and they will deflate as
your aux air runs down. Some have a check valve upstream from the
regulator in the engine compartment mounted high just inside the rear
doors so your bellows won’t lose air even if the aux goes down.
That’s in a perfect world of course. The bellows are a common source
of air leaks for the aux, usually leaking at the band clamp and will
drain your aux air in short order, especially if there is no check
valve.Aux air leaks!!!!! Welcome to Bus ownership, there is only one leak free bus in the US, right Hector?
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Check to valve and the regulator with soapy water for leaks, that Tom mentioned in his post. I had to replace my valve a tighten up the bolts that hold the regulator halves together. That fixed a big part of my aux air leaks.
A spray bottle of soapy water will help you find some of the leaks.
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It was suggested by Prevost tech at TX location to install a simple one-way valve in the mini air bag system. I’am sure if you contact Prevost you could get the right one that would fit.
Easy to install by the way, even Truk4u could do it.
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Mine also deflate quickly. If a check valve is installed near the air release valve in the engine compartment don’t you run the risk of too much pressure in the engine bay bags when the temperature rises? Air pressure increases with temperature. Bags that are inflated cold MAY be over inflated when it gets hot in the engine room. Without the check valve I’m assuming the pressure regulator will maintain a constant pressure.
Just a thought,
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The regulator pressure setting is more than likely going to reflect any anticipater minor pressure increases due to temperature rise. The issue relating to a check valve is that it must be ahead of the valve and regulator on the supply side so you do not defeat the belt tension release if you need to change a belt.
The belts should remain tight for days. If not there is a leak (duh!) and as Andre pointed out the regulator appears to be the biggest contributor, but it is probable the valve itself can be an issue. But then the air bags can leak and so can the fittings………..well, you get the drift.
Look up articles and posts by Hector. He is our leak guru and as he proves the rewards for chasing and curing air leaks is a coach that holds air for a long time. As to the seller saying “they all do it” what he means when talking about leaks is all the poorly maintained coaches do it.
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Gil, the one way valve is before the valve/regulator.
Another benefit to having this one way valve is on your pre trip inspection if the belts are loose your leak has been narrowed to that mini bags, regulator, or lines.
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The word is “bubble tight“!
The pneumatic systems in our coaches provides us with that luxury, smooth ride. But it is complex, dynamic and has many opportunities for leaks. My goal is to manage the pneumatic systems such that pesky leaks don’t become a maintenance problem. It take a significant effort to “tighten up” a coach in the age group such as mine, but the results are worth it.
The result of my quest for a leak free coach is I have learned much about the Prevost suspension system. It is very robust and manageable.
All the comments by others are good comments. My coach has an axillary air compressor that now runs so infrequently, I can not remember the last time I heard it run. I will check the axillary system pressure on occasions to assure it is at pressure.
So, it is possible to manage leaks on our coaches, but it does take determination.
The previous owner that stated that they all leak is partially correct. I believe it is most difficult to have a leak free coach, but much easier to have a manageable suspension system. I “tightened up” the suspension system on my bus and it lasted for three years before needing any attention. So, it does not require constant attention once repaired.
I use a hand sprayer, with a soapy solution as my main tool of choice. For the hard to find leaks, I use the famous Texas Bubbler. It finds them all.
I am currently in Upper Peninsula Michigan, Sault Ste Marie, Mi and the weather has turned cooler. High today in the mid 60’s and my pneumatic system is holding great. If you have any minor leaks, they will become major leaks in cooler weather.
Tom, Maybe we should have a leak finding party on your bus???
Good Luck,
Hector
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Damn, now I have to fix what I assumed was normal! I turn on the aux compressor before starting the engine to inflate the belt bags and all has been okay–or so I thought. Check valve to be installed on the supply side of the valve and regulator, get a spray bottle of soapy water…
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There might already be a check valve in the supply line side of the regulator / valve assembly on your coaches. It just might not be working, or the leak may be downstream from the check valve. With patience and a spray bottle it can be checked.
I have had a leaky regulator, a leaky valve (air leaked past the valve and out the exhaust port) and leaky belt tensioning bellows. Once you are confident you have no leaks, then if the belts go slack the problem can likely be the check valve.
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Your on Hector, I’ll provide the beverages.
Have you been to the Locks yet? Big fun watching the ships go through.
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LEAK!
First, if those engine belt retainer airbags are original, they are leaking.
The regulators are leaking.
the fittings are leaking.
Much of it can be tightened, but those air bags need replacement.
Now, as soon as you fix that, the next weakest link will fail.
Hector explained it the best.
I tried to go there and found I was loosing time from fun. About the only upside to it was I learned French as I remember distinctly being ordered by my dear wife to go and apologize to our neighbors for my French.
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Don’t get all charged up by just “beverages” from Truk4u.
Hector, if I supply the bus for “Leak Finder 101” here’s the menul,
Ice cold premium beer
Appetizers
Bone in rib-eye steak
Caesar salad
warm apple pie with a scoop of french vanilla ice cream
Rocking chair will be available for your short nap
Oh, almost forgot, your bus gets washed while dining.
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At various times someone asks; What should I carry in my spare parts? well here is a prime example.
The mini air bag. ONE SET for insurance.
Regardless of if you just replaced yours last week. Look in the engine compartment and notice. If your air bags leak, your belts will be loose (all of them) except for the alternator.
YOU WILL BE DEAD IN THE WATER.
If your belts are getting loose after a shut down even a lunch stop at a roadside rest get in the habit of opening the doors and making a visual for: all belts in place, until you fix the problem and then look in there often anyway.
JIM
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Well, I found two leaks in the auxiliary air system so far. The mini air bag (950208) under the fan gear leaks and the Norgren air filter assembly in the steering compartment also leaks at the gasket. Looking for a replacement gasket.
The aux air set up in the Marathon seems a bit strange. There is a very small air compressor with a separate pancake tank in the steering compartment. It is connected to the rest of the aux air system with valve with a warning label “Emergency Air Supply – open only if compressor fails”. I guess it is for “emergency” use only because of the diminutive size of the compressor.
If I open the “emergency valve” the little compressor kicks on and air flows from the pancake tank to the aux system (step slide, door etc). It does not inflate the mini air bags at the engine though. I would have expected the aux air system to air up the mini air bags. I can get them to inflate temporarily by using compressed air at the regulator fitting on top of the rear engine doors. Looks like the mini air bags only inflate when the main compressor on the engine is running.
Is that normal behavior for a Marathon conversion?
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That emergency fitting should fill the aux tank. You will also
have one in the port side rear engine door and that fitting will
put air to all tanks, brakes and aux. The one up front will only fill
the aux tank.The norgren filter has a small filter element inside, you should
be able to push up on the case and turn counter clockwise and the
canister will come off. You can get the filter at Grainger or
McMaster Car. I just did mine and had to replace it due to the metal
deteriorating.
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