GM 4108 Air Leak

[richard5933]

Went out today to air up the bus with the shop compressor and check on a few systems, only to hear a new hissing sound after I was done. It's normal to hear some suspension venting up front as things even out and find their level, but now I'm hearing a leak coming from the rear axle area. Oh oh.

I did the best I could to check things out with my limited abilities right now. I was able to slide the jack stands under the bus and slide under enough to listen more closely, and the leak doesn't appear to be coming from either of the brake canisters from what I can tell. Sounds much more like it's coming from the center of the bus, around the area of the air tank back there. Lots of other things that it could be, including what looks like miles of air lines and a few valves. Or maybe the tank itself.

I did my best to check for obvious cracked air line and didn't find any. Also checked the function of the brakes, and they appear to be working right now. The compressor can overcome the leak pretty easily at idle, so I'm pretty sure it can make it to the shop on its own, but it really isn't ideal.

Any thoughts?

[freds]

Plastic dip the entire bus?

[richard5933]

Plastic dip the entire bus?

Huh??

[chessie4905]

Does it change by applying or releasing the parking valve? I would suspect leaking out the bottom of the relay valve exhaust port. Bunch of valves there. All mounted to a plate. Relay, inversion, check valves, qr valves. I know because I rebuilt or replaced ever one of them 2 years ago. Nothing like a pit for that kind of work.

[buswarrior]

Take the bus for a drive, 2 hours, 100 miles, before emptying your wallet...

Sitting is no place for a bus and its air system.

On your return, report back that silence has returned to the Ponderosa?

Happy coaching!
Buswarrior

[richard5933]

Does it change by applying or releasing the parking valve? ... Nothing like a pit for that kind of work.

Best I can tell is that it leaks with the parking brake applied. With the brake released it doesn't appear to be leaking, at least not that I can hear.

However, I think I can also hear something when I depress the service brake pedal. Not sure from where though, and no one was home to help at the time. The brakes do work, so for now it's driveable.

I'm still recovering from surgery, so my ability to crawl around under a bus working on things is minimal. I'm thinking that if the shop can get it in this week I'll take it up there for them to put on the rack and inspect, at the least. It's about 40 miles away, so if driving will help things then maybe it'll be better by the time I get there. If not, then I'll let them check out and tell me what's going on. The guy there is familiar with DD3 brakes, so hopefully he can get things sorted out and diagnose.

Pit? Sure do wish I had one right now. Indoors even better.

[richard5933]

It's at the shop. They were able to get it in right away and hopefully it will be back home before the snow begins to stick.

Pressure held fine during the drive, but once at the shop and shut off the pressure dropped from 120 to 90 in just a few minutes. That would normally have take overnight to happen. Now the waiting begins, but at least it's there and not broken down in my driveway.

[chessie4905]

heres some pics of mine

[chessie4905]

more

[chessie4905]

if your plumbing is the same as mine, there are two relay valves. the one at bottom of vertical tank controls the service brakes and the one on upper left side controls dd-3s and lock ports.

[chessie4905]

due to your coach age, consider new dd3 diaphragms. Not that costly.

[richard5933]

The leading theory right now seems to be leaking diaphragms in the parking brake side of the DD3 brakes, at least in one side. Luke agrees. Guess we'll see.

If it is the diaphragms, I'll probably have the shop do both diaphragms on both sides and be done with it.

[chessie4905]

That is what was leaking on mine that got me into doing it all, including rebuilding the dd3's. Luckily yours is in nice shape with low miles.

[richard5933]

Yeah - the DD3 canisters themselves appear to be relatively clean and in good condition with most of the original paint in place. I'm sure that servicing them with new diaphragms won't be that difficult of a project.

[chessie4905]

Pretty easy actually. No giant spring to take your head off, just a lighter pressure one.. Good idea to lightly lube inside of clamp V's with some antiseize so they can seat nicely when tightening the little bolts, which, of course should receive some also.