What could cause this EMERGENCY

[diverman555]

I was driving my M923A1 about 2 miles from home, going about 50mph when I heard a loud PSST for about 5 seconds looked at my air gauge and it dropped to zero and the horn started blaring. Then I looked at my secondary tank gauge and it was dropping fast. Hit the brake pedal to see how the were and had a hard pedal that did not even start to slow me down, so I reached down and lifted the emergency brake some and it still worked. Looked at the secondary gauge and it was at 20lbs
so I started down shifting to slow. Then I felt the middle axle brakes start to lockup. I was about a half mile from home. In less than 20 seconds primary was empty and another 20 seconds secondary was at 20 lbs and worthless. Used trucks engine to out pull the locked brakes and the emergence handle to finally stop in the drive. I thought that's why there is a secondary and back flow valves if the primary fails the secondary will stop you. How could the primary then the secondary both fail so fast.
Going to manually try to fill primary and listen for leak and go from there. As of what I wrote you guys know as much as I do about what happened. Need some real good advise on this one. Never worked on air braked or used them before this truck, so I am starting in on the TM's but need as much good info as I can get. These are full of fail safes to prevent just what happened. So help me get started. Just does not seem that if a line broke this could have happened. Thanks Mark

 

[dmetalmiki]

did the horn continue or stop due to lack of prssure? Sounds like An "outside source" of air supply (as you suggested) would be a way to check for a MAJOR air line break?
Can't see the connection between the horn fault(?) and the power assist blowing out.
Please post again to inform what you find. Big safety point this.

 

[pctrans]

One of my 923's has a similar issue. When I release the parking brake, it loses air in both systems. But as to your issue, when your air pressure drops that low, the rear brakes will automatically apply, and stop the truck. You "should" be aware of that, having a CDL with air brake endorsement, to legally operate that truck. Hopefully this week, I will be able to determine exactly which valve is clogged/defective and will post up. Sounds like you do have a major air line failure in addition to a failed valve.

 

[NDT]

If you had your Class A or B or CDL with air brake endorsement, none of this would be a mystery to you. What happend is the normal sequence for a loss of air pressure event. Once the low air pressure warning goes off, There is just enough air to stop the truck quickly and pull over before the spring brakes lock you up in the middle of the road. Find what went "psst" and ifx it and you will be all good again. It will still be "pssting" when you reapply air to the system, either with shop air or the truck's compressor.

 

[73m819]

check for broken air line, try compresser to dryer

 

[ARYankee]

You may want to look at the dryer valve under the truck. Mine hung open and I couldn't build pressure. I crawled under the truck and used a screw driver to get it to close. I figure it was stuck due to sitting for so long. Worked fine on the trip home. Take a look at the M939 Series Anti-Lock Brake System Maintenance Sustainment Training Book ABS-MT-SH. It has some really good diagrams that will aid you with troubleshooting.

 

[porkysplace]

You really shouldn't use the word EMERGENCY in the title for a truck that's broke down in your driveway. This should be reserved for members that are in need of immediate assistance that are broke down on the road .

 

[diverman555]

Well got it figured out what happened to all systems. Read the TM's to see exactly how things were put together. Pulled out a 3/4 plug on the side of a tank built a fitting with a shut off valve and an air chuck. started pressurizing the tanks and heard the pssst. Come to find a hole in the bottom of the muffler, where the exaust came straight down to the 4 pressure release cockpit valves just over the tool box. The heat of the exaust heated the plastic lines and when the first line blew a hole in it, the pressure went to the next line causing all of them to fail at once because they were hot and soft. That caused all systems including back up to fail. Found 5 blow out holes in the lines right below the muffler. Going to replace all lines tomorrow.
Thanks for everyone's help.

 

[diverman555]

Found problem never would have guessed

Well got it figured out what happened to all systems. Read the TM's to see exactly how things were put together. Pulled out a 3/4 plug on the side of a tank built a fitting with a shut off valve and an air chuck. started pressurizing the tanks and heard the pssst. Come to find a hole in the bottom of the muffler, where the exaust came straight down to the 4 pressure release cockpit valves just over the tool box. The heat of the exaust heated the plastic lines and when the first line blew a hole in it, the pressure went to the next line causing all of them to fail at once because they were hot and soft. That caused all systems including back up to fail. Found 5 blow out holes in the lines right below the muffler. Going to replace all lines tomorrow.
Thanks for everyone's help.

 

[diverman555]

As for the CDL, this truck is designed where upon failure of the primary system, the secondary system is supposed to give you 5 brake pedal stops before it runs out of air, unless you have a multiple failure like this cause by one system failing after another. This type of failure is not even in the TM's.

 

[porkysplace]

As for the CDL, this truck is designed where upon failure of the primary system, the secondary system is supposed to give you 5 brake pedal stops before it runs out of air, unless you have a multiple failure like this cause by one system failing after another. This type of failure is not even in the TM's.

But exhaust leaks are part of the daily pre-trip inspection.

 

[nf6x]

did the horn continue or stop due to lack of prssure? Sounds like An "outside source" of air supply (as you suggested) would be a way to check for a MAJOR air line break?
Can't see the connection between the horn fault(?) and the power assist blowing out.
Please post again to inform what you find. Big safety point this.

I think that he was referring to the low-air alarm where he wrote "horn".

It's true that checking for exhaust leaks is part of the pre-trip inspection, but after reading this I think I'll go over my M923 when I get it (this week, I hope!) to find any places where plastic lines are routed near exhaust, and consider adding some sort of shield or insulation to protect the lines from a hot leak. That, and I'll check the exhaust pipe for signs of rust, and repair any weak spots that I find.

Thanks for sharing the details of your air system failure, diverman! I'm glad that nobody got hurt. Brake system failures are very dangerous!

 

[quickfarms]

The primary pressure is for the wet tank and the secondary pressure is for the service tank

Modern air brakes incorporate 3 systems.

The service brakes are used for normal driving.

The low air buzzer, is this the horn he is talking about, will come on between 55 and 75 psi

The emergency system should apply the brakes between 20 and 45 psi to stop the truck using a relay valve.

The spring brakes apply as a last resort. Since you said only the second axle locked up you should check the third axle to see if it has maxi cans on.

The compressor should cut in at no less than 85 psi, most cut in at 100 psi. The compressor should cut out at no more than 130 psi, most cut out at 125 psi.

There is also the applied brake test that checks for air leaks.

Most of this is part of the cdl pre trip

This is another example of why you should have a cdl with an air brake endorsement.

 

[chevyCUCV]

As stated, this is standard operation in getting your airbrake endorsement

 

[engineer233]

Not all states require a CDL for these trucks. Kentucky is one of those states as long as the truck is tagged for farm use.

 

[lankyevilme]

Even if you don't have to have a CDL, you have to have an air brake endorsement to run a vehicle with air brakes. I have a non-CDL but had to get an air brake endorsement to legally drive my MV - This poster showed us why.

 

[maxxplanck]

Mark, on Feb. 26th you stated this quote
"I think the mods should tell everyone do not use help in your title unless you are broke down on the side of the road. Help is for an emergency's and someones oil pressure gauge not working is not an emergency. I scroll through new post's when I get on and when I see help I stop their first, to make sure no one is broke down by me and needs help. Just my thought's on help"

You just broke your own rule

 

[quickfarms]

What is the GVW of this truck?

 

[pctrans]

check for broken air line, try compresser to dryer

Not ALL the 900 series trucks have a dryer.

If I'm not mistaken, the GVW is around 33K.

 

[EO2NMCB]

I've hauled them on my step deck for the Govt, that said they weight 25,000 lbs give or take empty for the the 900's. It has to have a GVW over 26,000. By Fed rules one should have a class B cdl.