923a2 air brakes leak

[roofconsult]

I just got my 923a2 last week and love it. Everything checked out good, so we have been driving it under controlled conditions to get used to it. Yesterday I noticed that I could hear air while holding my foot steady on the brakes. A lot of air. So I watched the pressure gauges and found the pressure to drop very quickly when on the brakes. It seems like the secondary pressure drops even faster than the primary. It goes from 120+ to below 60 in about 10-15 seconds when holding the brakes down. I haven't gotten someone else to hold the brakes so I can listen yet, but I do know that I hear the air coming from the snorkel. I have educated myself some on the brakes and plan to get more educated. But in the meantime, does someone know what this means?
As always, thanks a bunch.

 

[quickfarms]

Do not drive it until you find the leak.

If you attempt to drive it the compressor may not be able to keep up with the leak causing the emergency brakes to apply and eventually the spring brakes. Picture a panic stop without any input from the driver.

Since it only happens when your foot is on the foot valve the first place to check is the glad hands on the bumpers, the air cans, the foot valve. The best way to find the leak is to charge the system and shut off the truck, then have someone depress the foot valve and try to find the leak.

The air system can not leak more than 3 psi when the foot valve is depressed for 60 seconds when the engine is off.

You should perform a pre trip and cola test before you drive the vehicle every day.

This is all covered in the CDL training and test.

 

[73m819]

I just got my 923a2 last week and love it. Everything checked out good, so we have been driving it under controlled conditions to get used to it. Yesterday I noticed that I could hear air while holding my foot steady on the brakes. A lot of air. So I watched the pressure gauges and found the pressure to drop very quickly when on the brakes. It seems like the secondary pressure drops even faster than the primary. It goes from 120+ to below 60 in about 10-15 seconds when holding the brakes down. I haven't gotten someone else to hold the brakes so I can listen yet, but I do know that I hear the air coming from the snorkel. I have educated myself some on the brakes and plan to get more educated. But in the meantime, does someone know what this means?As always, thanks a bunch.

Yes, you have a AIR LEAK

 

[roofconsult]

OK, thanks. I was just hoping that since I hear the noise from the snorkel that someone might have run across the same problem and could help pinpoint.

 

[roofconsult]

I did check the glad hands and looked at the cans which are intact and lines intact. Haven't checked the pedal valve yet......will do so tomorrow.
Thanks

 

[Csm Davis]

Maybe a diaphragm, I had one go out on my wrecker on the way home today. But you need to trouble shoot the system, as what one truck sounds like doesn't mean that's what is wrong with yours. If you would like more help most of these guys will be more responsive if you have read the -10 and tried some of the trouble shooting first. Also try searching for problems in other threads that are similar to yours.

 

[quickfarms]

I did check the glad hands and looked at the cans which are intact and lines intact. Haven't checked the pedal valve yet......will do so tomorrow.
Thanks

The air cans have a rubber diaphragm inside of them that can tear and cause a leak.

Be very careful with air cans. The maxi cans have a large spring inside of them that is under tension and should not be taken apart.

 

[wreckerman893]

If the cover is off the glad hand and the valve is open the air will rush out when the brake pedal is mashed down.

 

[Suprman]

It sounds like a service brake system leak if it only leaks with your foot on the pedal. There should be no air going to the spring brakes when the brakes are applied.

 

[roofconsult]

Thanks again everyone. I have walked around the truck with the engine killed and my wife holding steady pressure on the brake pedal. I can hear the air, but it is not coming from the cans or the glad hands. The noise comes from the snorkel. I'm baffled.

 

[RustyM923]

The discharged air from the cans (diaphragms) is discharged to the air intake. Listen closely to each brake chamber...it's likely a torn or delaminated diaphragm inside.

 

[Suprman]

It is probably venting thru a torn diaphragm in one of your service brake chambers. This is not something you will be capable of repairing yourself unless you have special training. Do not attempt to take the brake chamber apart. Take the truck to a repair shop that deals with heavy vehicles. It should not be an expensive repair.

 

[roofconsult]

Rustym923- Do you know where the brake discharge air dumps into the intake at?
Kind thanks to Rustym923 and Suprman. I think y'all hit the bullseye. As always, if I can figure out how to ask the question right, someone here knows the answer.

 

[plumkrazy]

X2 regarding the brake chambers. Some years ago in my life as a Big Brown feeder driver, a newby mechanic in our district lost is left thumb, left forefinger, and nearly his life when the chamber blew apart. Those chambers are a bomb waiting to go off--'nuff said.

 

[doghead]

The fact is, you can safely change your service brake diaphragm, but not your spring brake chamber diaphragm.

If these terms are unfamiliar to you, then you should either educate yourself more before working on your brakes, or take it to a truck shop.

Here's some good info. http://www.zafr.com/trucktcom/b_chmbrs.htm

 

[RustyM923]

Rustym923- Do you know where the brake discharge air dumps into the intake at?
Kind thanks to Rustym923 and Suprman. I think y'all hit the bullseye. As always, if I can figure out how to ask the question right, someone here knows the answer.

At the back side of the air intake. You need to get under there and look...

 

[roofconsult]

Thanks everyone for the help, but I am still stumped. I still need help with my air leak problem in my brakes. It is a large leak that occurs when I apply pressure to the pedal valve. The leak gets bigger as greater pressure is applied. The only place that I can hear the leak is in the snorkel and in the air line where it enters the intake tube.

As some suggested, the first thing that I did was to check the glad hand valves, which were all closed. Then I shut off the engine so that I could hear and listened and felt each of the brake chambers on all three axles to see if I could detect leakage past one of the diaphragms while my son depressed the pedal. I could not hear any air leakage.
I know it is on the signal side of the brakes as no leakage occurs when there is no pressure applied to the pedal. And it seems to drain the secondary tank immediately, with the pressure drop in the primary tank lagging behind.
If anyone reading this has experience with this problem, please respond. Or, if you know someone that knows everything about the brakes in the 923A2 please forward to them for response.
As always, thanks for the help.

 

[Suprman]

You need to take it to a heavy truck repair shop. There are 2 brake systems on these trucks. Spring brakes the air keeps the brakes off and when you step on the brake pedal it takes the air away and the super strong springs apply the brakes. God forbid you have some catastrophic system failure the brakes will apply and the truck will stop. Service brakes are applied with air sent when you step on the brake pedal. They do not have springs they are applied with air pressure against a diaphragm. If you step on the brakes and air is leaking, then more than likely it is thru one or more of the service brake chambers. The air vents out thru the stack. Take the truck and have someone that knows exactly what they are doing fix it. I wouldn't attempt to fix air brakes myself.

 

[quickfarms]

It sounds like one of your service brake diaphragms has failed. Unlike a civilian truck the military trucks vent the air thru the intake stack. With the brake pedal depressed you need to listen to each can with a stethoscope to see if you can hear air moving. If this does not work you need to start opening the vent lines until you find which can is leaking.

 

[goldneagle]

I had the same problem on my M923. Ended up being 2 torn diaphragms on the front brakes. The diaphragms are $5 or less at NAPA. Will need to be ordered. It's easy to replace the diaphragms on the front brakes. Just drain the air from the system. Disconnect the two hoses. Remove the clamp that holds the 2 halves together and remove the diaphragm. Install new diaphragm and reverse the steps to reassemble. Do not mess with the spring brakes on the rear axles. (the ones with 2 cans on them) If they don't look identical to the front ones do not mess with.