Chasing Air Leaks

[MyPrevost]

I mixed a good shot of dishwasher soap into a full squirt bottle of Windex, and it created a very foamy and sticky solution. It worked quite well, though it takes a few minutes for the soap to dissolve in the Windex.

I suspect there might be an air leak in the one-way check valve located between the compressor and the first tank. I can see it clearly and it’s easily removable since it’s on the top tank. Beyond that, I’m not sure where else to look.

I run the bus until the air dryer reaches 120 lbs, then shut it off, with the brakes set. Despite no audible noise from any fitting, valve, or airbag (I’ve thoroughly soaped everything up), I can’t find any leaks. Yet, my air pressure drops at a rate of 1 lb per minute, which is significant and should be noticeable. It’s frustrating to be in a position where I’m troubleshooting a problem like this!

I won’t feel satisfied until I resolve this issue.

While I know the engine air compressor can cause problems, it’s not very common. I’ve also ruled out the parking brake valve and tag dump switch; both were replaced last year and have passed multiple soap tests.

I think I’ll check the valve first since it’s inexpensive. Do you have any other suggestions?

I’ve noticed a wiper control bubbling slightly. Additionally, I recently replaced the low air warning light sender in the steering bay, which was also bubbling, and I found one loose fitting in that area. The wiper control can wait until Monday; it’s likely not the main issue, which I suspect is something more serious.

[MyPrevost]

Hi Joe,

It seems you have a noticeable leak.

First, if your bus has the emergency brakes engaged, the entire braking system will have zero air pressure, so you can rule that out as a source of the leak.

If you can get your coach inside, there’s a chance you might be able to hear the leak more clearly. Unless there’s a leak in the steering compartment or engine area, it’s challenging for me to detect the leak when the coach is outside. If you can’t find any soap bubbles indicating a leak underneath the coach, I recommend using a stethoscope. They’re inexpensive, and starting with a funnel will help you pinpoint the general area of the leak. Then, you can use the narrow tube to isolate the leak further. If that doesn’t work, consider borrowing or acquiring an ultrasonic leak detector.

I agree with your suspicion about the check valve; it sounds reasonable. A stethoscope should help you listen for internal airflow, which could save you from purchasing a replacement if it turns out not to be necessary.

Since your coach isn’t a typical conversion, there may be unique areas to consider. Our conversions have various potential leak sources, such as sliding floors, pocket doors, bed lifts, air locks, belt tensioners, air horns, etc. Unfortunately, these won’t be detectable from underneath the coach.

[MyPrevost]

The only pneumatic components in the bus are the passenger floor and the air door lock, both of which are sealed tightly. And let’s not forget the Dave Matthews switch!

[MyPrevost]

I searched thoroughly on my coach and replaced the air bags, discovering one pinhole leak. Even after that, I couldn’t identify the source of the problem; I was losing air pressure down to zero in just 4 to 6 hours.

However, with the help of A1’s ultrasonic leak detector, we found the leak in the front steering compartment within a minute. It turned out to be in the two Norgren valves, which didn’t bubble or produce any audible sound. The air was escaping through a hidden route in the valves and was only detectable with the amplified sound that only an ultrasonic leak detector could pick up.

After replacing the two Norgren valves, the issue is now resolved. It was frustrating and time-consuming, but I’ve learned what to do next time.

Best of luck to you in finding your leak without the use of a detector!

Roger that!

Vehicle Details:

2008 Liberty DS XL2
2023 Denali Ultimate
My 6th Prevost

[MyPrevost]

Hi Joe,

I was experiencing significant air loss as well and discovered a major leak in the small regulator for the belt tensioning bags located in the engine bay. To find it, follow the air line from the bag to the little regulator, which is mounted above the engine bay doors. You’ll need to stick your head inside and turn around to see it.

I’m not sure if your make and model have the same setup, but it’s definitely worth checking out.

I believe Tom was the one who initially pointed this out to me, and he was spot on!

[MyPrevost]

Joe,

Since you have OTR air, it’s important to note that you also have pneumatic belt tensioners and the regulator that JJ mentioned.

Given the rate at which you’re leaking, you might even be able to hear it if you open the rear doors!

However, I believe that’s auxiliary air, so it’s unlikely that a leak in that area would cause the same drop in pressure as your primary system.

[MyPrevost]

Hi Joe,

Make sure to apply plenty of soap to the fittings on the step slide and regulator. The step slide on my coach was a pain to access, but I suspect those pesky Norgren valves might be the culprit.

I recommend using kids’ bubble soap, as it’s really effective for revealing leaks.

We’re pulling for you!

Best,
Tom Chilcote
Trans-Specialists RV Batteries
tom@transspecialists.net

[MyPrevost]

Joe,

Diagnosing issues online can be pretty tough, but I wanted to share a personal experience from my previous coach that might help.

While driving down I-81 toward a long downgrade near Hillsville, VA (where it always seems like something goes wrong), I noticed the air pressure dropping rapidly. I quickly pulled off the road, and just as we parked, the bus sat down flat—zero air.

I connected an external air supply to the coach at 110 psi and listened closely. The air was escaping from the curbside tag brake chamber, which I believe is a Number 50. It turned out that the brake chamber had failed, with the diaphragm hissing like a disgruntled mother-in-law who disapproved of me from the moment I took her daughter away.

Since I’m not very familiar with all the mechanical stuff, I grabbed a Vise-Grip, clamped it onto the input air line to the chamber, and wire-wrapped it tightly to some other components underneath. I started the engine, and the bus came back up; the air held, and we drove it to Florida to have it properly replaced by professionals.

I know this was a Non-POG-Approved-Stupid-But-It-Worked (NPASBIW) repair. It’s not something one would typically expect to fail while on the road, but it happened.

Despite the NPASBIW fix, I still have the Vise-Grips and am saving the used wire ties for Jon, as he likes to keep extra parts on hand to fiddle with when he’s bored. Now, if only I could remember to give those ties to him—that’s CRS, not NPASBIW! Wait, since Mango Bus Lines are PP’s, maybe I should give the ties to him instead? Hmmm.

Good luck to you!

 

[MyPrevost]

Jeff,

That’s a peculiar situation.

On our bus, the chamber on the tag axle is not a maxi but rather a service diaphragm. When you release the parking brakes, no air goes to it; it functions solely as a service chamber. The only time air flows through that line to the chamber is during braking.

Did they install maxi chambers on tag axles in some model years? If you discovered the leak while the bus was aired up and it leaked just sitting there, then it sounds like you may indeed have a maxi on the tag.

Here’s a tip: Sometimes, when a maxi brake chamber begins leaking emergency air, it can be tricky to diagnose. Often, when this happens, the brakes on the opposite side of that axle will drag harder. This might occur because as the air tries to flow past the leak to the other side, the volume decreases even more. I hope that makes sense. Additionally, a maxi can sometimes leak at the valve feeding it rather than at the chamber itself, and it may even send air back through the emergency brake button.

Jeff, it’s always preferable to eliminate a leak by any means necessary rather than risking brake failure. Whether it’s with Vise-Grips or zip ties, I’ve even heard of people taking the line loose from the fitting, putting a dime in the end of the hose, and reattaching it for a perfect seal. Duct tape and tarp straps could be a fitting name for an old bus!

In my opinion, that’s the best course of action if a repair isn’t possible.

As for our step setup, it’s located in the passenger armrest attached to the wall, which isn’t difficult to access at all. The step has an adjustable regulator, and if I turn it all the way up, it kicks the step out so fast it could decapitate a small dog!

Just a thought!

[MyPrevost]

Joe,

One of the guys at POG II had a great idea for finding leaks. He began disconnecting air lines from the auxiliary manifold until he was able to isolate the source of his leak. If this method doesn’t lead you to the leak, at least you can confirm that it’s not in the auxiliary system.

Best,
Mike

[MyPrevost]

Joe,

I might be completely off base here, but I wanted to share this anyway.

On the ’93 chassis, the component I disconnected was reportedly for activating the tag axle lift. However, you can usually tell a guy who doesn’t know much when he believes just about anything!

[MyPrevost]

Joe,

Haha, I know! It’s all good. You know, Henry Ford was illiterate, and you’ve heard how his court case story goes. We all have each other’s backs!

I’ve been working with heavy equipment and trucks my whole life, but I’m struggling to find what I’d consider a 5 or 6 on a scale of 1 to 10 for a 1 lb per minute leak on the bus. It’s not great—almost embarrassing!

Yesterday, I think I actually managed to give Jon some helpful computer tips for his pictures. Unbelievable! How did that happen?

[MyPrevost]

Debbie has taken the grandkids to Tennessee and has been gone since Friday morning—if anyone hasn’t noticed! This has given me the opportunity for some personal time with my bus.

The colder weather has worked in my favor for detecting air leaks, as it’s been in the 30s. I’ve spent the better part of two days working on it and, as it turns out, I’ve found plenty of leaks.

I discovered very slow leaks at the wiper control, low air warning sensor, and both small diaphragm valves inside the tower above the left drive air bag, as well as at the A/C belt tensioner shaft.

Additionally, I found two loose fittings—one in the steering bay and one on the engine—that are slightly worse. Most significantly, there’s constant air escaping at the exhaust port for the ride height valve on the right rear, which is where the majority of the air loss was occurring. YeHa! Success!

I feel much more confident knowing I’ve thoroughly inspected the bus with a soapy squirt bottle from front to back three times. The other two height valves were replaced early last year, and now all three will be new. I can’t wait for tomorrow morning so I can call Elgin!

[MyPrevost]

I installed a new ride height valve, which has significantly slowed my leak from 1 lb. per minute down to 10 lb. per hour. That’s much better, but it’s still not good enough.

I’m currently waiting for the remaining valves and the belt tension cylinder rebuild kit. If those don’t resolve the issue, my newest best friend, Orren, has kindly offered to send me his ultrasonic leak detector via UPS so I can locate the small leaks I can’t hear.

Thank you again, Orren!

[MyPrevost]

It’s been 12 hours since I built up the air pressure and shut it off, and we’re down to 50 lbs now. For the first time since we bought it a year ago, the suspension is still up the morning after!

I found some issues at the belt tensioner/regulator and the corresponding air lines.

Additionally, I’m aware of two small diaphragm valves at the drive axle that are bubbling. Those were included in yesterday’s parts order, and I expect they should reduce the remaining leak by at least half, if not more.

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