923a2 air brakes leak

[roofconsult]

Thanks again to all of you for the advice. I think y'all are right, it has to be a diaphragm that I can't hear at the chamber because the vent goes to the intake tube. I understand about the power of the springs in the spring brakes and fully respect a loaded spring (more dangerous than a loaded gun because there is no scope and crosshairs on the loaded spring). From your advice and reading more TM's I see that the next step is to isolate which chamber is leaking. Removing the vent lines at the cans and testing each one individually looks like the most reliable method.
It looks like changing a diaphragm is a simple task if done properly, even if it is in one of the chambers that is piggybacked with the spring brake. I have read the TM about caging the spring brake before removing the clamp on the service chamber. Has anyone here actually done that?

 

[quickfarms]

You can cage a spring brake can and replace the service diaphragm but you are betting you safety on two little tabs on the caging bolt.

With the number of caging bolts that I have seen pop out over the years I would not risk it. The caging bolts fail for three main reasons. The ears on the bolt shear off.

The socket in the can fails. The most common one is the socket inside of the can is dirty or corroded preventing the bolt from fully seating.

Personally if one has failed the rest are not to far behind unless they have been replaced recently.

It is much safer just to replace the whole can with a new one

 

[roofconsult]

Amen brother. Thanks for the educated advice. Understood sir!

 

[Floridianson]

I have done them and I just make shure and hope the person who had the truck kept the dust caps in place. If the caps have been kept in place and no dirt or mud then no problem. I just make shure when installing the caging bolt it turns a 1/4 turn by hand then tighten the nut down.
Just watch yourself and stay to the side and don't have your face in line with the can cap. It's realy no problem just think smart.

I rather change out the can rubber than stay out all night drinking then face the better half in the morning.

 

[roofconsult]

Thanks for the education.
Bring the better half with you

 

[quickfarms]

I have done them and I just make shure and hope the person who had the truck kept the dust caps in place. If the caps have been kept in place and no dirt or mud then no problem. I just make shure when installing the caging bolt it turns a 1/4 turn by hand then tighten the nut down.
Just watch yourself and stay to the side and don't have your face in line with the can cap. It's realy no problem just think smart.

I rather change out the can rubber than stay out all night drinking then face the better half in the morning.

The problem is that this is a surplus truck and you do not know the history and weather the plugs have been in there since new or were open at some point and replaced.

 

[Floridianson]

Yea you never know what you get in a box of cracker jacks. Myself if the cage bolt feels like it locked in correctly I do the job.
If you are not comfortable doing it as said have someone else do it.

 

[Hainebd]

They are nothing to mess with. If you are not experance day changing them, have a truck shop do it. It is one of those jobs that are easy to do but can realy ruin your day. I swear that spring is 3 foot long in that can. It can and has done damage and pain when the chance comes along. My uncle was explaining to me how to do the brake rubber and took clamp off before caging. I thing I diff a few then just replace entire can. Can is $50 rubber is $10 each plus labor. It is about equal.

 

[juanprado]

This is not a 3o/30 can. I don't think you will find one of these for $50!

 

[srodocker]

I have the same problem. Will take these ideas and tips in mind whenever I get around to getting outside to work on it.

 

[roofconsult]

What an adventure it has been troubleshooting the brakes in my 923A2. When I first noticed the leak I wanted someone here to give me a quick answer to the problem so I could hurry up and get back on the road. A glad hand leak was suggested so I hoped that was it because it would be quick and easy. Then after listening to other suggestions I was convinced that it was a torn diaphragm in one of the chambers. It made sense because that would vent to the snorkel, which is where I heard the noise from the leak.
So I started studying the TM's really hard. I learned all about the wedge brakes that these have, and the function of the primary, secondary and spring brakes. I learned about caging the spring brakes and was prepared to change the diaphragm even if it was on one of the piggybacked chambers.
So on Saturday, with the help of my granddaughter, I started out to determine which of the chambers was leaking. I methodically took each vent line loose from the chambers one or two at a time and had her step on the brakes. We checked each and every one of them and they all tested good........figure that.
So then I found which line going in to the intake tube is the brake vent line by listening and feeling while she held the brake down. Then I took each of the lines loose one at a time from the tee in the line about 2' away from where the line goes in to the intake. I found that the leaking air was coming from a line that went to the front of the truck. So I figured I must have mis-diagnosed a leak in one of the front chambers. But I decided to trace the line to where it went, which was not easy. My truck was rebuilt at the Blount Island Depot and everything was painted green, including all of the bundles of wires and hoses.
I then found that the vented air was coming from the front limiter/release valve that is mounted on a bracket attached to the frame cross member about 1' behind the front bumper. By taking the lines loose from the valve I determined that the valve had to be bad. So I took the valve apart and found a nylon ball that seats in the valve was broken.
So I studied some more. I found that the valve that I had does not match the valve shown in the TM for my truck.
This made me wonder why did someone put the wrong valve in my truck. So then I read the TM about the retrofit of the ABS in these trucks. These did not originally come with ABS, it was added later. There I found that the valve was changed during the retrofit to the Bendix LQ-2 valve to apply 67% of the braking pressure to the front brakes to balance the system for the ABS.
As bad as I wanted to have my truck on the road now, I could not find it anywhere within a 150 mile radius from here. So I ordered a new one off of Epay for about $28. It will be in this week and I'll be back on the road and smiling again.
A few good things came from this adventure:
1. The willingness of those on this website to help out and share their knowledge was confirmed.
2. I now have a really good understanding of the brakes on my truck. I don't know every valve and tee, but I do know my way around in the system.
3. My grandbaby knows the difference in a 3/4" and a 13/16" wrench.
Thanks to all for the help and support.

 

[doghead]

Fantastic!

I hate to admit it, but I wanted to suggest/mention that a relay valve may be the issue. (too late now)

It is great that you have it solved and gained knowledge and understanding of your truck. That cannot be gained with a quick answer in a forum.

Good for you(and your granddaughter)!

 

[roofconsult]

Me, my baby and my grandbaby.

 

[srodocker]

congrats on finding the problem. Hope to find mine today as it needs to get sent off to the fabricator soon eeekkk.

 

[quickfarms]

I am glad you found the problem

If you do not know this already, you are required to perform a COLA test on an air brake vehicle every day as part of your pretrip inspection.

The applied brake test is especially important since you have opened up the airlines.

This test involves testing the governor cut in, governor cut out, low air buzzer and applied brake test. We also test the parking brake.

If you google it you will find multiple outlines explaining the test.

This is how we do it. It is a little out of order from the book but it tests all the functions. Start truck let it build air pressure. Do your walk around pretrip. The pressure should stop building around 120 psi, this is your governor cut out. Depress brake pedal, release and look at pressure gauge it should start climbing very slowly around 90 psi, this is your governor cut in. Now rapidly press the brake pedal until the low air buzzer goes off, thus should happen between 55 and 75 psi. The next test is the applied brake test. Let air pressure build back up to the max and shut the truck off. Depress the brake pedal and hold for one minute, during this time you should not loose more that 3 psi for a single vehicle, 4 psi is you have an air brake trailer attached.

The final test that we perform in the parking brake. This is simple put the vehicle in gear before releasing the parking brakes and if the vehicle move you have a problem.

It is common for the air to leak down overnight to the point where the low air buzzer comes on but if this happens in a shorter time, like a lunch stop, them it is time to find the leak.

 

[roofconsult]

Yes, COLA is now our standard procedure.

 

[jwesley74]

We had a similar issue with our 925. It was dumping air thru intake when parking brake was released. Had to cage spring brakes just to drive it. Service brakes seemed to work. Finally discovered problem, it was the double check valve under drivers step. Piece of sand got in there and stop the plunger from sliding back and forth. Cleaned/lubed it and reinstalled it. Problem fixed.

 

[roofconsult]

That's great to know. Thanks for the info.

 

[roofconsult]

I know you had a time troubleshooting.

 

[Tommy Frantz]

This a little off thread, but I'm having problems with my 923a2 intermittently rapidly leaking down its primary air supply. The secondary stays good as long as I don't draw much air. As soon as I rev the truck up, that air comes back up to 120 +- psi and sometimes stays there fine; other times it drops back down to about 20 psi if I let the truck idle. I've walked around the truck and I hear no leakage, and when I parked it a little while ago the air stayed up, even at idle. Any ideas?

Thanks for letting me hi-jack the thread.
Tommy