View Full Version : Air system overhaul

05-11-2014, 04:10 PM
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. 'Sorry 'bout such a long-winded post.

05-11-2014, 05:58 PM

I hope Joe Camper jumps in here. Based on his posts, I'd say he's the goto guy for everything air related (not his only area of expertise). My own coach leaks down in a similar fashion to yours, although it takes a lot long for it to leak down. My gut feel is the problem is between the primary and secondary tanks and the air compressor. My Air Dryer has been rebuilt, so I'm hoping I can rule it out. I checked my parking brake push-pull valve and it doesn't seem to leak.

My problem is being able to comfortably and safely get under the bus. When you figure it out, let us know what you discovered.

Joe Camper
05-11-2014, 09:58 PM
What exactly r u trying to achieve. Have the brake tank pressures stay up OR keep the back of the bus from squatting or leaning?

The bigger leaks first smaller leaks next that's how Id go. Don't overcomplicate things any more than they already r.

05-11-2014, 10:28 PM
Gil and Joe, I replaced the element and check valve in my air dryer, but wonder if there's a problem there just based upon the singing noises that are continuous inside it. I think I want to try to rule it out by capping the output of the wet tank, pressurizing just that part of the system and seeing if the singing stops and the pressure holds. I replaced the parking brake valve, but I know I've got aux leaks at the driver's seat and door. Those, however, are not the major leak that is drawing the system down so fast. I'm thinking I want both the brake tank pressures to stay up AND keep the back of the bus from squatting. I'd like to know the air dryer and brake systems are okay before tackling the aux leaks. I'm thinking I can isolate each to get that answer. Is that a foolish idea? I've got a set of jack stands that let me slide under the coach safely.

Joe Camper
05-11-2014, 11:11 PM
OK so the back is squatting that means the aux air is leaking out. Doesn't matter where U start go for the brake tank leaks instead if u want first.

That's major pressure loss in short order should be able to slow that by better than 1/2 before havin to jump through hoops.

Best way to start is ...........to just start.

05-12-2014, 07:36 AM
Ron, I had a similar sound from dryer that I heard when I shut the engine off and it would sing until the pressure dropped to about 90psi. It turned out to be the turbo cut off valve. I watched the tech replace it and purge valve and dessicant filter. In hindsight I should have had him replace the check valve to just because the dryer was sitting on the bench.

05-12-2014, 11:43 AM
Gil, that's just the kind of information I was looking for! Thank you. As I hit the books in an attempt to understand the system better, I may be getting a little smarter. The sounds I'm getting from the AD-9 dryer have convinced me that one or more of the five valves in it are leaking. On my first rebuild years ago, I only replaced two. With leakage at that end of the system and my seat and door cylinder leakage at the aux end, all it would take is check valve leakage at the secondary tank to bring the entire system down (I may be wrong again, but at this point, I'm assuming that the primary tank check valve is the delivery port check valve in the air dryer). That theory may be somewhat weak in that most of the time all three gauges come down together when I would expect them to drop at different rates. On the other hand, there are times when the secondary tank stays at full pressure for several hours after the other two have dropped to zero. Pinpointing multiple simultaneous malfunctions is like balancing three balls on each other, but with luck, I will prevail.

05-12-2014, 06:43 PM

If there isn't a check valve at the input to the main and aux tanks, then a bad check valve would deplete all 3 tanks at the same time until the aux protection valve kicks in. With that said, I can't believe there aren't check valves at the input to both primary and secondary tanks. Lacking check valves at the input to primary and secondary tanks would not provide tank redundancy.

Good luck and keep us posted,

Joe Camper
05-12-2014, 08:37 PM
"On my first rebuild years ago, I only replaced two."

Air dryers R relitivly inexpensive and should be switched out every 24000 miles anyway get a rebuilt with a core exchange for yours instead of rebuilding that again.

If u remove the steel braided line from the compressor that will tell U if anything air is escaping out. That hose is very usually very difficult to remove due to the tremendous amount of heat and heat cycles that part endures. U may or may not be able to get it off to see. Depends how good U R Any time replacing a bendix engine air compressor be sure to have new fittings and hose U will be glad U did.

So id start there with a new rebiuilt dryer

Here is how I find hard leaks. Was doing this today between the rain in Michigan. I scheduled a local rental a steam cleaner. Not some garden hose pressure cold water thing a 3000psi with a kerosene burner to cut crud and grease. This bus would loose brake tank pressure slow and sure till it got to 30 or so where it would stop all in an overnight.

My ears aren't what they used to be. This leak was in the bus when we bought it. Ive looked at it numerous times before. Always been able to just here a slight faint of a noise of air from someplace but never pinned it down.

Well..........its day has come.


Here is a GREAT idea for safety. See the 3 sets of stands the bus is propped up on.

Well for saftey sake after I have set the bus on just the body stands dumped all the air bag pressure to get the tires and rims off DO THIS.

Build 2 more sets of stands 1 set 16in high for the tag and another 12in high for the drive axle.

After removing the rims and tires lower your bottle jack to the additional jack stands. Then start the bus in Level low and put air back into the left and right corners NOT to the point where the body starts to come off its stand. Stop before that.

If U put air back into the suspension like this what is going on is now we have the weight held up by 3 points not the 1 body support point that U would otherwise be working with.

If U R at home U can very safely get away with crawling around under your bus with the 1 cause U know what U have.

Ron U should do this if u ever really get crawling around under yours I know U r on the road be sure of the base u r supporting the body with. Do this it is much safer in a foreign location.

The extra stands also make it very easy to get to difficult air leaks. U can safely a and completely air the suspension up off its body supports to look for leaks there WITH THE TIRESAND RIMS OFF.

Once U get here all those other valves that concern U Ron..................now u can pull um out badda bingbadda bang.

Don't mess with rebuilding dryers swap it out for a complete rebuild.

Joe Camper
05-12-2014, 08:44 PM
Getting back to chasing leaks.

After u pull the tires and rims steam it.

200 bucks with a trailer so I did not need ramps.


3 or 4 hr got a entertainer with 640000 miles cleaned up great. Well worth the money. Now its gonna be way easier looning for the leak. This bus chassis has not been this clean for 500,000 miles. Look at those footballs I think they r original they r almost mint.

Here is proof if u service the cassis right it does have the ability to go a million.

Joe Camper
05-12-2014, 08:47 PM
Then I went back at it again with this.


This is the stuff dreams r made of when looking for tough ones.

Joe Camper
05-12-2014, 09:00 PM
So after going in again over the same bus a few good times already U do it again and a bit harder and better and U get it.


Bull's-eye. tough to see but the small 1/4 in plastic signal line to a brake relay valve over the tag is bubbling relatively "much". In my opinion if its not all the current loss it is most of it.

This is how u will get your bus tight Ron.

Unless U R either Very good or very lucky a bus like yours will probably need this drill at least 2 or 3 times before it gets real tight. 2nd and 3 times r a sinch really. U will learn and also find a lot of other "stuff" I guarantee.

I just redid a charter chassis with the kneeler like yours. It had all 3 axles knor bremse disc brakes and still had its straight axle up front. Not sure of the year 96 or 97 I think???????

That kneeler had 3 norgrens up front to make that happen. 1 Three position and 2 Two position spool valves. That's for a straight axle without level-low aint that something. Just to get it to kneel.

U also have 4 2 position norgrens on the back of yours for the tag lift only 2 of which can make u lean.

Come to Chicago and park at my house for a while let me help u if U need.

Joe Camper
05-12-2014, 10:57 PM
Look how clean it came. No soap just hot water at 3000psi that's it. If I used the syphon and shot a little aluminum brightener on this it would be BAM. Should run to Napa and get a gallon.


A great by-product of looking for a tough air leak.

Joe Camper
05-12-2014, 11:15 PM
Ron before U replaced the yellow parking brake button how was it leaking. Constantly or only if u grabbed it and wiggled it around?

05-13-2014, 12:50 AM
Gil, I learned the wet tank check valve is indeed in the air dryer even though the Prevost parts manual exploded view doesn't show it (but it is listed). The primary tank valve is at the forward end of that tank. I'm sure the secondary air tank has a check valve as well. I'll find it later. Hands on discovery sure beats book learnin'.

Joe, buying a rebuilt air dryer sounds like a good idea. Prevost Parts wants $500 or so, but others get about half that, however, none tell me if the desiccant cartridge is included. It's hard to believe Prevost wants $130 for the cartridge while NAPA is listing theirs for $26. Is it included with the rebuilt AD-9? Do you have a favorite source? That's a great tip about disconnecting the braided line from the compressor to check for leaks at that point. I didn't have a problem disconnecting it at the compressor when I replaced the compressor, or at the dryer today. 'Guess I got lucky. Thanks for the tip.
Wow! You really know how to jack a coach up for inspection and steam cleaning! I'll do the same if I ever get a level concrete floor to work on. How impressive! I cleaned my engine and transmission with a big steam jenny and degreaser once. It sure looked great! I'd love to do the entire underside as you have. At $110 per quart, the Bird Dog leak detector liquid must be really great stuff! How does it differ from the Dollar store's bubble soap? Mine is a '98 with all disk brakes and a straight axle. I found one of the kneeling Norgren valves frozen and just bypassed all three of them, disabling the kneeling circuit. My parking brake valve leaked primarily when I wiggled the knob. Why do you ask? It seemed prudent to replace it since they're not that expensive. I'll stop by the next time I'm near Chicago.

Joe Camper
05-13-2014, 06:14 AM
The parking brake button is something that is often overlooked and often leaking. On most buses very difficult to remove. My XL had to have the seat pulled and still I had to cut open an access panel to get it out as were most of the XLVs right thru the 2000's.

Wiggling it will expose a bad valve often long before an audible leak appears on its own.

The bird dog detector is like half cooked jello in the bottle. Barley thin enough to squirt. It stays put and does not evaporate quickly at all. It is just the best of a bunch of different products ive gone thru.

That price is not right I for sure do not spend that. I get it by the gallon for 20 or 30 bucks I think.

dale farley
05-13-2014, 08:47 AM
Has anyone tried this leak detector. The HVAC group (80%) use this, and it is supposed to bubble up to 30 minutes after application. Rather thick stuff but does a good job. The tech who gave me this bottle said it was less than $10 at HVAC supply houses. I see there are 5 places in Pensacola that sell it.