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Air system overhaul
Having replaced all air bags and brake chambers, my next project is a complete overhaul of the air system. Mine was a 1998 model seated coach, built in 1997, so it’s time. I’ve found surprising evidence of poor maintenance while it was in commercial service, so I’m assuming no component has been replaced that didn’t fail completely. It is long overdue for replacement of all Norgren valves and probably much more. Two years ago, after finding black oil in the air from the air dryer bleed at the rear fill port and a few tablespoons of what looked like chocolate milk at the wet tank drain, I replaced the engine-mounted air compressor and governor along with the air dryer element and check valve assembly. I’ve never found any moisture when bleeding the accessory air tank, but I did find a Norgren valve in the kneeling circuit frozen and a trace of oil at the exhaust ports of all three ride height valves. I solved the frozen Norgren valve problem by disabling the kneeling system (installed new air lines from the front air bags direct to the front ride height valve). If a lady can’t step high enough to enter the coach at normal ride height, she’s too old for me anyhow. 🙂 Now I’m ready to troubleshoot the entire system and replace components as necessary. Without a leveling system of any kind, my coach air system should be simpler than most.
The air system bleeds down almost completely within a couple hours. A single major leak that depletes the system that fast should be easily heard, but with my loss of hearing from a career in aviation, it would just about have to blow my socks down as I walk around the coach for me to notice. What I can hear is the AD-9 air dryer singing continuously. A professional medical stethoscope against the side of the canister indicates the sound from the system charging, steady-state or leaking down is coming from inside more than at the valve at the bottom. The system passes a DOT pre-trip check if I’m doing it right, but when I air the coach up to 115 PSI, then bleed the wet tank, the gauges show 110/110/105 (F/R/A) by the time I get to the driver’s seat. A half hour later, they’re at 60/60/60 and after an hour, 30/30/40.
Now, after all this history, here are some questions the group can perhaps answer for me: What’s the best approach to take when troubleshooting the system from scratch? Since I can’t tell if the sound the air dryer is making is caused by multiple leaks beyond it or air bleeding backwards through it, is it reasonable to begin by capping off the supply from the wet tank to the primary tank and airing up the system? It seems like that would test the integrity of the compressor, governor, wet tank and air dryer. If that was successful, I’d reconnect the primary and cap off the delivery port of the secondary tank to eliminate the entire aux system and see if the system still holds pressure. In my mind, that would take all aux leaks out of the equation and point the finger at pressure protection valves, check valves or brake system leaks. Only after making the heart of the system leak free would I then start on the Norgrens and the smaller leaks. Am I on the right track? I have no experience working on the air system, so I’ll appreciate any guidance or suggestions any of you might offer.
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22 Replies
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Ron,
Here would be my path to addressing the issues. It appears you have decided to repair / replace the air system. Since that is the objective why not disregard the leaks for the moment and create your list of things you are going to repair and replace and then just do it. Before you start however, try to establish a priority list such as doing the full DOT brake check, and if that shows all is OK then start on the side with the greatest potential for leaks which is the accessory side and replace valves, air bags, fittings, etc. that are on your list of things to do.
Then follow that with your brake system. Brake chambers, hoses, fittings, relay valves, protection valves etc.
Or reverse the sequence above. Then when you are done finish up by chasing the remaining leaks.
As long as you are breaking into the system and are replacing things you will be repairing or eliminating some leaks anyway, and some things not leaking could start leaking because they did not get screwed in tight enough.
Then when you are done with the replacements you should have a tighter system with fewer leaks to chase.
My only concern would be to look for anything common to the three systems because they are all leaking down at once so be especially alert to bad protection valves or check valves between the tanks. Make sure where you added your compressor hookup that is not contributing and that you have a check valve at that conection point.
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Ron,
I too have a hearing loss. When I decided to go after air leaks in our Marathon other PC members had electronic air leak detectors.
In checking around WW Grainger had a good selection. The one I zeroed in on had sensitivity settings for the head phones plus a strength increase light bar when the sensor picked up a leak.
I questioned $400 bucks but gave it a try. Good investment saving lots of time and frustration.
AL
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If all three gauges are leaking together I would look at tag lift valve it will drain the aux air then the parking brake valve it will drain the prime and secondary gauges down together easy to replace and cheap try that first then move on unless you can see bubbles when you spray it with soap the back of the gauages leak bad also but try the other first even if all protection valves are good and check valves , those two things will drain the gauges down
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Al, Thanks for the Grainger tip. I was surprised to find they have TEN leak detectors to choose from. Which model do you recommend? I agree, there’s a real advantage in having the right tool for the job.
Jon, I’m sorry if I failed to make it clear that I’ve already replaced all the air bags and brake chambers. I haven’t, however, used my copy of Hector’s bubbler to be sure none have leaks. The front air bags stay up for a week or so, but I can’t be sure about the rear ones since the tags dump their air when the aux system drops below 30 PSI or so. Your shotgun approach of simply replacing all the components I’m planning to replace anyhow, then seeing how much of the problem they solved certainly has merit, but would deny me the joy of being able to put my finger on the biggest culprit when I find it. I intend to replace all the Norgren valves, but hadn’t thought about simply doing a full replacement of all the other components without determining if they’re leaking. You’re right though, that would be doing the job right and would not waste time looking for leaks until there should be none.
I did the full DOT brake check and found the leakage to be within tolerances, but barely.
I may have overlooked them, but I can find no check valves physically on the primary or secondary tanks or the lines going to them. The pneumatic schematic doesn’t appear to show one until the pressure protection valve (PR-2) and the shuttle-type double check valve (DC-4) in the front service compartment. That’s why I thought it might be prudent to isolate the compressor, wet tank and air dryer, check them for leaks, then add the primary and secondary tanks (with the brake system), test again and finally add the aux tank. If the air dryer is the major problem, that would be obvious right away. It’s one thing that would affect all three systems, isn’t it? Next, I’d find out if the braking system had a major leak and finally know that any remaining leaks had to be in the aux system. Other than knowing this building block approach will take more time, do you see any obvious errors in my thinking? Go ahead, lay it on me! I can take it. I’ve gone off in the wrong direction many times before. I have the utmost respect for you pioneers who’ve been through this and know the best way to proceed. My shop compressor quick disconnect has a little leakage, but I disconnected it from the coach as soon as I had the system aired up, so it shouldn’t have been an issue.
Thanks again for your help. -
I re did my air system in the 1997 royale it would hold air in all three gauges for months there are lots of things to check start with some of those things spray soap and water first . I would put some ball valves in to seperate the aux from the other two systems
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Wow! Peter, you may be onto something! I had a raccoon chew through the right tag lift nylon air line near what I believe is the tag lift valve above the left main wheels. I replaced the line, but have heard a hum coming from that area as the pressure is dropping after I shut the engine down. I’ll move a house battery for access and see if capping the supply line makes a difference. It took me a while to discover the critter’s handy work, so a high volume of air passed through that valve whenever I raised the tags, possibly causing corrosion. When I found the severed air line and the wire bundle he cut through, I thought I repaired it all, but he may have damaged the supply line as well. It’s well hidden and hard to reach, but I’ll check it much more closely and let you know what I find. I recently replaced the parking brake valve because it was leaking, but haven’t checked the new one lately. I stopped the leaks at the gauges once, but I’ll check them again as well. I know I’ve got seepage at the driver’s seat and my air actuated door, but the problem I’m searching for is larger than any of those leaks. The fact that the low air pressure alarm goes off at the proper reading on both gauges at the same time as pressure is rising tells me the gauges are not grossly off in their readings. Thank you for your suggestions.
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Just so all are aware Ron’s coach was formerly a seated coach and his air system has substantial differences compared to the typical motor home conversion shell.
Ron…I forgot we talked about those items being new. Another senior moment.
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Ron,
You’re on the right track. I like to isolate the systems and then test each one. That’s what I did with my air bags and was the only real way to track down the leaker by isolating the bag and pink tank.
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Ron,
Grainger, Catolog page 877, part number 6JHN8 , Bacharach Ultrasonic leak detector.
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Ron.
I find the Prevost air drawing for my coach most difficult to read, drawing D140210, Revision M. I don’t see check valves on the supply lines to the Primary and Secondary air tanks, but I have to believe they should be there. Maybe there’s one on the output of the dryer, even though from a safety aspect it would make more since if they were mounted at the tanks.
Although this drawing and information isn’t specific to Prevost and doesn’t include a lot of the air system we have, this is a good drawing and troubleshooting guide for the main part of any air brake system. http://www.meritorwabco.com/MeritorWABCO_document/tp9973.pdf
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Jon, If I could get a handle on brevity, I’d stand a much better chance of not losing folks before I finish a way-too-long post. Thanks for plodding through anyhow.
Peter, I’ve definitely got ball valves in my plan. I’ve lusted over Jon’s since he first showed them to us. A system holding air for months is the kind of stuff dreams are made of!Tom, Thanks. I’ll have a pretty simple suspension system when i finish pulling out all that old unnecessary tour bus stuff.Al, ‘Good info. That may be the perfect set of bionic ears for me!Gil, Thanks for the drawing. That helps! If I hang around here long enough, you folks may get me eddicated yet! I’m sure I’ve missed at least one check valve that has to be there. I will find them all because I feel they may be responsible for much of my leak problem. I pulled one apart next to a ride height valve and found trash in it that may have kept it from seating properly.This is a great place to go to get important information I hadn’t considered and to reduce the number of stupid mistakes I would surely make as I slowly climb the learning curve. Thanks to all. -
Ron,
I’ve been thinking about your comment about check valves and without looking at your coach diagrams or parts list I think you have to have check valves either attached to or as part of the protection valves.
You have to be able to retain air pressure in your systems even with a failure in another system. If your primary has a massive leak for example then air needs to be prevented from flowing into it from the secondary. And vice versa.
But you can verify if those components are OK by bleeding air pressure from the tanks in sequence in both directions.
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You’re right, of course, Jon. Thinking back, I confirmed that when I aired the coach up, shut off the supply, bled the wet tank and the system held pressure (at least for a while) after the wet tank went to zero. The valves are there; it will just take me a while to identify them. I will temporarily install a gauge on each tank to check that the valves are holding pressure. Thanks for the follow-up.
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What has me confused is it appears your three air system pressures track one another suggesting something common to all three is leaking. The problem I have is I cannot think of a single leak point that would cause that because the protection valves (if functioning as intended) will not allow a leak somewhere in one system to drain air from the other 2 systems.
I think verifying they function is a very good step.
FWIW I have drained the primary tank and then got interupted, and when I returned a day later the secondary pressure had not dropped. Similarly I have bled air pressure from the aux system and the protection valve retained pressure in the secondary and primary tanks but not full pressure.
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The tag valve at drivers area will drain down the aux , and the park brake will drain down the prime and secondary together
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