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PNEUMATIC SYSTEM MOISTURE
Over the years Prevost Conversion pneumatic systems have been an active topic for discussion. With the introduction of slides maintaining the pneumatic system has taken on significant importance. Leaks have always been discussed, and as coaches age leaks become increasingly more troublesome. But of far greater importance is the need to keep moisture from the pneumatic system.
Because of the need to keep slide seals inflated to keep the coach weather tight, the coach when traveling or stored outside is going to have compressed air in the pneumatic system. When air is compressed by the engine driven compressor or a converter installed auxiliary compressor it is carrying the moisture we call humidity. That moisture has to be removed before it enters the coach pneumatic system. When the source of compressed air is the engine driven compressor preventing moisture from entering the system is done by the air dryer. The air dryer, if maintained does an excellent job of removing moisture and a dryer that is properly maintained effectively stops all moisture from going beyond the dryer. The timely replacement of the dryer cartridge or media along with periodic draining of the air tanks is all that is required to maintain and verify the condition of the pneumatic system.
Draining the tanks in sequence verifies there is no moisture in the tanks, and also is a means of checking the operation of the protection and check valves essential to the system. The interval between air dryer cartridge replacement is determined by how often the coach is used, but for most of us 2 years is more than adequate. As compressors age they tend to pass oil and oil carried with the compressed air is going to shorten the life of the cartridge so if moisture is found in the wet tank, especially if it has oil in it the interval for replacement should be shortened. Oil coats the dessicant and its ability to retain moisture is comromised. When oil is seen on the coach frame near the dryer “spitter” it is a sign the cartridge change intervals need to be shortened or the compressor is starting to get tired.
The chassis system, as good as it is, is offset by the converter added auxiliary compressor and a usually less capable water or moisture separator or trap. Instead of moisture entering the system from the engine driven compressor via the wet tank, brake tanks and accessory tank, the aux compressor can introduce moisture via the accessory tank. The amount of moisture coming from the auxiliary compressor can be substantial. Worse, the system protection installed by converters is no where near the quality of the Prevost air dryer so owners have to be especially vigilant to prevent the introduction of moisture into the coach pneumatic system. It may even be necessary to modify the original system to insure moisture coming through the aux compressor is trapped and goes no further. This is especially true if the pneumatic system needs to be pressurized so slide seals can protect the coach interior.
The big deal about preventing moisture from entering the pneumatic system is how it affects the coach and its working parts. At the extreme, but not unheard of, has been reports of air bags filled or partially filled with water. At the very least that affects the ride or handling of the coach and can impact safety. But once a coach has water in the systems to that extent it is almost a sure bet that the complex pneumatic system and its components are damaged. The accessory portion of the pneumatic system is everything but the brakes. That means every part of the coach that uses compressed air, from the suspension and all of its valves to slides to converter installed devices such as floor slides, pocket doors, bed lift, the driver’s seat, etc are slowly being damaged or destroyed.
Keeping the air in the systems dry is critical, but not difficult. All it takes is a good maintenance program and attention to details.
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9 Replies
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Good reminder Jon!
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Good reminder Jon….
Here in my humble opinion this is the worst thing bus owners can do…. found under drivers day.
seems like no big deal,
what would it hurt
my compressor won’t run every hour
just open that valve and my problems go away
When you leave this valve in the open position you have now combined AUX air and your sealed Prevost air
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Al, you have slides. Unless your slide seals are fed from your Marathon aux tank, or your Prevost accessory system is 100% air tight you need pressure on the Prevost accessory system. There is one thing worse than getting moisture in the Prevost pneumatic system and that is getting moisture into the bus via leaking seals around the slides because once you get mold and mildew you have problems far bigger than you can imagine.
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Jon,
I have 3 Valid slides aired through the Marathon AUX system. I’ve been chasing air leaks, found many which is at or near 3 day’s for full air discharge. Check valves hold air in seal on all 3 slides tight.
I like the Valid slides, less parts, less noise. Unless I develope a leak pretty much trouble free.
AL
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Just wanted to make sure. Owners of coaches with slides need to know if the coach is stored outside or is outside for extended periods that inflatable seals need to be inflated and remain inflated.
There cannot be check valves at seals because when the slide has to move in or out the seal has to be deflated so if there is a check valve it has to be beyond the valve that exhausts the air in the seal.
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Jon,
Some of the benefits of Valid seals. One, increase in slide width over Prevost , Valid calls them super slides. I have not measured but at least 6 inch increase width +/-, over Prevost slides.
I’ll get some photo’s showing the Valid control panel.
In response to your post…. Valid vacuums air out of each slide before moving. Check valves hold seal tight unless the actual seal has a failure.
Valid Slides are new to me but having gone through Prevost locking pin 101 school, replacement of Prevost salon seal at $4000 in my XL-II, Valid seems to have found a better way.
AL
1999 XLV X-6872, liberty #440 -
Well to quote that philospher, “Truk” …
“Who needs sticking slides” 🙂
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Al, your slides may have check valves but if they are in fact in place they are not going to be positioned to hold pressure in the seals because as you mention there is a need to vacuum the seals to move the slide.
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