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Air Issues
Just stop by Prevost JAX for my annual service. In additon, I asked to have the air dryer serviced, the rear valves checked for leaks, as the coach was sagging in the rear and for the front air bags to be checked for replacement.
Two valves and a check valve were replaced in the rear, the air dryer was serviced and the front air bags were replaced.
Upon leaving Prevost, I noticed that the two brake air gauges were not in sync. One, the “R” was just above 60 psi and the other “F” was usually at about 125 psi. Pumping the brakes got both guage to re-charge to 125 psi but the “F” continued to drifted out of sync.
That afternoon we parked the coach and levelled. In the morning the front was all the way down and the back, which had been sagging prior to visiting Prevost was still a the height of levelling.
I air’d-up the coach and head three hours home. The dash air gauges once again were out of sync.
While attempting to park the bus, after a ten minute engine shutdown to unhook the toad, and with very light pressure on the brake pedal (engine idle enough to back the coach) the low brake air alarm sounded and the emergency brake set. I went to fast idle, air’d-up and finished parking.
Prevost advises me not to drive the coach until they can get a tech to inspect the coach. Prevost JAX suspects the air dryer.
Any idea of what went wrong at Prevost JAX?
Mike
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10 Replies
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Multiple things come to play, as the front air bags will stay up no matter what the air pressure does, if fittings at air bags and norgren valves (tank also depending on year) or something like the norgren valve is leaking…
As far as the air and the air dryer there are a few things that could be going on….when running or immediatly upon shut down do you hear air leaking near dryer area
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Brake air and aux (suspension air) are completely separate. They tweaked something wrong. Do not drive as they suggested.
Jeff and Lindsay
91 Marathon XL -
Haven’t had time to listen for air leaks. Plan on airing up, shutting down, key in on position and then listening to air dryer and front of coach. Will do this in a couple of days, however, my aging ears are not air leak sensitive. Have asked wife to loan her ears; more sensitive. I’ll keep you posted.
One of my problems is that the rear used to fall and the front stayed put. Now the front falls and the rear seems to stay put. Prevost swapped rear valves (seems they succeeded) and front air bags (seems they failed).
The air gauges have me concern. I agree with Tom, must be at least two different problems.
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It is probable the reason the front is sagging down over time is a leaking fitting. The original fittings are the push in type and not compression fittings and based on the age of coach those could be the leak source. That is easy to check by turning the front wheels all the way and squirting soapy water on the fitting at the top of the air bag. An overnight leak will blow a lot of bubbles.
But as has been said the suspension leaks have absolutely nothing to do with the brake system leaks.
When it was mentioned that the front and rear (secondary and primary) systems are not the same I did not think “air dryer”, but did think “protection valve” or something related to the check valves in the brake systems. The front and rear should be filled simultaneously but when air is discharged from the tanks it can be at two different rates because of the valving that is preventing total air system pressure loss. Why they are not filling equally might be from the valving downstream from the wet tank and not the air dryer service being the cause.
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Went to coach today and air’d it-up. Shut the engine off and left the key in the on position. Heard no air leaking at air dryer, within driver side rear wheel well (raised fender skirt) and front airbags. I sprayed all visable fittings in these areas. However, I was unable to see any air line fittings to front airbags. I had the wheels turned. I may have been looking in the wrong place, though (towards the top, not the bottem of the bags).
Jeff and Lindsay
91 Marathon XL -
Mike, It seems obvious that one of the service tasks was not done correctly. You mentioned that they changed a check valve. Do you know what valve that was or where it was located. Someone more air savy than me may be able to evaluate if that could have been put in backwards causing your symptoms. I had a ride height valve replaced by one of the luxury coach dealers in Tennessee and they plumbed it backwards causing it to exhaust continuously at ride height. Jeff quickly diagnosed it and reversed the lines.which took care of it. Good luck. Jeff
P.S. It seems it would have to be a huge audible air leak for the engine compressor to not keep up. When my height valve lines were reversed there was a rediculously loud hiss that I could hear from 100 feet away. I drove 8 hrs to Jeffs like this and the compressor kept up no problem even at idle.
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Mike,
When a coach leans or sags the leak is relatively easy to isolate. It is most probable it is going to be from the Norgren valves that “lock” air into that specific section (front, left rear or right rear) to the air bags within that section, including their fittings.
If you chase leaks it could range from an internal leak of the Norgren valves to leaks in the fittings to leaks in the air bags themselves to leaks in all the associated fittings to a leak in the “ping” tank or its fittings, including a petcock that is not fully closed. With the fenders up and a shop mirror with a long extension you can see almost 100% of the above without getting under the coach. One item sometimes overlooked is if the Norgren that controls locking air in the air bags is leaking internally it may require an ultrasonic leak detector to “hear” the leak, or may require checking the exhaust port on the 3 port Norgren up front that controls the exhausting of air bag air.
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I’ll add a small detail often over looked by many leak chasers….use a thick soapy spray.
Finding leaks with soapy water requires some patience. Spray your suspected fittings then walk away…..in 15 minutes take a look. Often, you’ll be amazed finding a mountain of suds around threads or line connection.
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what Al said, add some Karo syrup to the soapy water so it will stick to the fittings and valves long enough for the bubbles to form. Don’t spray it on and watch it run off.
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The problems have been fixed. 1) The air dryer started failing at approximately 80 PSI. Replacement fixed this problem. The coach falling in front problem was fix by tightening a fitting. The coach has remained up without sagging for a week so far. Prevost sent two techs to my place to fix these issues. Nice that they came to me but not good that they let the coach out of their place with quality control issues.
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