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A New Problem
I have been away from the bus and civilization for about two weeks but came back to discover a new problem.
When the bus is running and I am prepairing to level up and park, I flip the lever to lift the tag to set the back of the bus down. Normally the air releases, the tag lifts and the back of the bus lowers.
Now I hear a lot more air when I flip that lever and after the bus sits down the air keeps right on “escaping” and won’t stop until I flip the lever back in the proper position for driving.
In that position everything seems to be OK and the level is right when driving and the air pressure remains good when driving or parked.
Any ideas about what has changed, what the problem is or what the solution might be? Am I in danger of losing air while driving?
Thanks in advance….
DKO
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30 Replies
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check the 2 three port Norgen valves between the tag and drive axles on each side, check the valves and brake chambers that lift he tag and you are likely to find the extra air noise is coming from them or associated air lines or fittings.
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Can I see any of that without getting under it? I have no way to get under it.
DKO
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If the problem is a failed diaphragm on the brake chambers that lift the tag axle you can see where the noise is coming from on those. But if the leak is associated with a three port Norgren of the valve controlling the lifting of the tag you need to be beneath the coach to see those.
If you can raise the coach up and support it at the rear lift points it is possible to slide under and sit up in the area of the tag and drive axles. But please do not ever attempt to slide beneath the coach unless it is supported, ESPECIALLY if you are looking at a potential suspension problem.
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I had a similear problem. I would hear a large escape of air when I would lift the tag. It was a busted diaghram in the can that lifts the tag. I was able to remove the can and replace the diaghram with a $70 replacement from NAPA.
Turned out to be an easy and cheap fix.
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That’s the part your can see DKO. Andre may have had the problem you have.
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If it is not raining too hard tomorrow, I plan to start the bus and see if I can spot the leak for certain. Where exactly is the can that you think may have the busted diaphram? You say I should be able to see it?
Thanks,
DKO
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I just fixed the same problem and Jon is on point with the 3port valves not shifting position either port or starboard. You will find air coming out of the exhaust port on the failed valve with the tag lift valve actuated.
Good luck.
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DKO,
The lifting device for the tag is visible behind and above the tag wheel. It lifts the axle via a short length of chain. The device is nothing more than a brake chamber. I can’t remember if it is a 30 or 36 chamber, but if that is the leak the chamber is available at any truck parts store for $35 or $40 or you can repair it by buying the rubber diaphragm for about 25% or less of the cost of a new one.
And Gh is right. if it is the three port norgren(s) and it fails to completely shift the spool it will also exhaust air, more than just that in the tag air bag..
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DKO,
If you look above the tag tire, about the 11:00 position, you can see the lift can. If it’s leaking, very easy to hear the leak or you can spray it with soapy water. The 3 ports are mounted high above the frame and can’t be seen unless you’re under the bus.
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I started the bus and checked for the leak today. The rush of air is definitely on the drivers side of the bus. This is a picture of the canister that is hooked to the chain that lifts the tag.
I sprayed it with soapy water and had my wife flip the lever inside and you can see it is leaking some in two places.
However, I could not feel any big rush of air escaping when I had my hand in there. Of course I could not leave my hand in there for long because the bus is coming down. I did try it several times in different places around that canister but I could feel no air with my hand. The canister is obviously leaking some but I do not think it is the main culprit.
Does this mean the air escaping is more likely coming from the norgren valve underneath the bus.
If the valve is exhausting with the tag lift engaged but at no other time I should be OK to drive it, correct?
Do truck shops work on these valves?
Thanks,
DKO
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There should be no leaks at the lift chamber. The foaming you show indicates the rubber diafram inside has deteoriated. Be very careful when opening and closing the chamber. Inside you will find a compressed spring that can cause severe damage to anything close by if it gets away from you.
While the process of rebuilding the chamber is quite straight forward, it is not as easily done as it would appear.
Compressing the two halves together with the new rubber in exactly the correct position and then installing the clamp to end up with an assembly that is leak free, requires some degree of luck.
New chambers are quite expensive when compared to other sizes due to the fact that they are not used in as many applications.
I recommend buy them from Prevost and they will last you 15 plus years.
JIM
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Not sure I completely agree with Jim. If this was a combo such as the service and emergency brake combined I would never consider repairing or servicing it due to the potential energy in the emergency brake spring.
However the lift chamber for the tag axle is a single service brake chamber, and while it has a spring inside, the spring is relatively light only serving to retract the plunger. The two halves of the chamber, with the diaphragm held between the two halves can usually be held together with one hand while the outer band clamps are put in place with the other hand. As to the cost however. definitely determine the generic size which will be a 30 or more likely a 36. A 30 is cheap, sometimes on sale at truck parts stores, sold out of a large bin for as little as $30. The 36 size is less common, but still relatively inexpensive. If the choice is to replace the diaphragm (looks like a black rubber cereal bowl) most are available for under $10.
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I would never service that part due to risk. And it is not worth the risk…period. Take Jons advice on the Prevost parts replacement and while your at it do the other side at the same time.
Both likely have been in the same amount of use..
AL
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Al,
That brake canister can easily and safely be rebuilt by replacing the rubber diaphragm.
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Thanks for the help, guys. It sounds like we have some disagreement as to the prudence of trying to repair this myself and I will consider all of your input. Thanks much.
I have not been able to put the bus over a pit so I haven’t laid eyes on the valve underneath to see if it is leaking as well.
There is one more fact I have discovered that may shed some light on the subject. I moved about 10 miles yesterday and when I parked I left the tag in the drive position, leveled the bus and shut it down.
I usually lift the tag before leveling because it will drop anyway in 20-40 minutes when my air pressure goes down. I did not lift the tag and 20 hours later the tag has not lifted and the bus has not lowered in the back. That has never happened until this air problem.
Does that indicate this may be a valve problem? I mean, I like the fact that the tag is holding but it is not normal for my bus but it is definitely holding now.
Any ideas?
DKO
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