M923a1 air brake question

[Farmallmilitary]

Hi, i am new to the forum and new to the military truck world. A few weeks ago i got a m923a1 truck and it works fine and drives around like it should but whenever i am at a stoplight waiting for the light to turn green the air pressure starts dropping fairly rapidly. It only happens when i have my foot on the brake and if the light is long enough then i have to use the parking brake and let the air build back up. I cant believe this is supposed to be how it works because it is a pain. The truck builds pressure fine and maintains it fairly well but once you have you foot on the brake it would probably empty the tanks in 1 to 2 minutes. I am guessing a valve of somesort is leaking. Just letting you know i have no past experiences with air brakes so dont be surprised if i ask a dumb question.
patrick

 

[cornrichard]

Make sure the gladhand shut off valve is off. You could be supplying air to an invisible trailer. They use a lot.

 

[Farmallmilitary]

Yeah i checked that but wouldnt that mean it couldnt build air at all if the gladhand valve was open? Also is there a central gladhand shutoff valve if so where? I know there are four different gladhands on the truck, one at each corner of the truck. Is there a master valve for the gladhands?

 

[nf6x]

There should be a shutoff valve on each of the two rear glad hands. If the one on the passenger side (the emergency connection) is open, then that will keep the truck from building pressure. I think that having the one on the driver's side (the service connection) open will only bleed pressure when the brake pedal is pressed, so the truck will still build pressure.

There isn't a central shutoff; just a ball valve on each of the two rear glad hands.

 

[Farmallmilitary]

Thanks i will check that in the morning. If its not that what else could it be?

 

[topo]

You may have a bad diaphragm on one or more of your brake pods . air it up push the brake peddle and have some one listen to the brake pods on each wheel or even could be the quick release valve .

 

[seano11]

Yup bad brake pod

 

[rosco]

The truck is not safe to be driven on the highway. The issue needs to be resolved, before its returned to service! One of the very good reasons to require a CDL, is that it mandates a thorough understanding of the air brake system. A DOT spot safety check will result in a substantial fine.

 

[m16ty]

You may have a bad diaphragm on one or more of your brake pods . air it up push the brake peddle and have some one listen to the brake pods on each wheel or even could be the quick release valve .

This is my suggestion also. It's fairly common for a diaphragm to give out. One word of caution- it is possible to replace the diaphragm without buying the whole brake chamber but you better know what you are doing. There's a spring in there that can KILL you.

It could also be something as simple as a line. It could also be a relay valve or some other valve leaking somewhere. As said above, have somebody push the pedal and listen for the leak (shut the engine off so you can hear better).

 

[M35A2-AZ]

I would think it maybe a line that is leaking. I would turn the truck off and push on the brakes and see if you can hear a leak, also get a soldier B to help out.

FYI, if your truck has a ball valve on the front service glad hand, Do not close it, it will cause brake problems. I know first hand.

 

[Farmallmilitary]

Ok i went and checked the gladhand and it was closed, no air leaking there when i pushed the brake. I figured out when i push the brake and hold it, the air being released is coming out of the air intake tube at a constant speed. So i think that cancels out the idea of the brake pod. I am guessing it is the quick release valve like said before.Also this morning i drove it around and for some reason the parking brake was not releasing fully. I tried turning the fine adjustment knob at the end of the handle but still no luck. I dont think it did this before but for some reason today it was very noticible.I am pretty sure there is a coarse adjustment right at the brake drum on the driveshaft. Also why is there an air actuator for the parking brake? What does it do?

 

[seano11]

Don't wanna sound like a a-hole but for everyone's safety go get your air brake ticket. You will understand the braking system on your truck a million times better. The air actuator on your brake lever is to set your air brakes. Still sounds like a bad pod since all your air lines are plumbed into your intake stake.

 

[nf6x]

Getting an air brake ticket is highly advisable, but it still won't answer all of the OP's questions. When I took my CDL training and passed my test, the training included the dash-mounted air brake controls used in common civilian semi tractors, and didn't cover anything like the combination air brake + mechanical driveline brake in the M939 series. It covered testing to see whether there is an air leak, but not how to diagnose where the air leak is. So the OP's question about the actuator on the parking brake lever would still be unanswered by the CDL training.

The parking brake lever on the right side of the driver's seat does two separate things:

1. The lever itself applies a mechanical parking brake in the driveline, just like in older MVs with air-over-hydraulic brakes.
2. The lever bumps that air valve behind it to control the spring parking brakes, which are RELEASED by air pressure, like on any normal air brake truck.

Since you need to be able to have the spring parking brakes released in order to tell whether the mechanical driveline parking brake is adjusted correctly, there's a round button to push on the dash to release the spring parking brakes. With that button pressed in, you can turn the knob on the end of the parking brake handle to adjust the mechanical driveline brake until it holds the truck. That knob is a fine-tuning adjustment to let the operator take up minor wear, and there's also a rough adjustment under the truck that needs to be in the right ballpark. The dash knob is only used for testing/adjusting the mechanical driveline brake, and it is left pulled out at all times in normal operation.

If air leakage can be heard from the engine air intake, then I've been told that's probably from leaking seals in the brake pedal valve which are allowing air to leak into the vent system. I have the same issue on one of my trucks, but I haven't dug in there to fix it yet (don't worry, the truck isn't going on public roads until I fix that).

 

[M35A2-AZ]

It maybe the treadle valve or one of the check valve.

 

[Farmallmilitary]

Thanks for the input and i am planning on getting my cdl and air brake certification hopefully being 16 wont be a problem. Right now I am just trying to resolve this problem that it is having, I am used to tractors and cars not military trucks so i am working on the understanding of this big rig. Where is the treadle valve and check valve located on the truck.

 

[nf6x]

The treadle valve is the valve body that the brake pedal is part of, under the driver's side of the dashboard. I don't know where the various check valves are, though. I haven't dug that deeply into my M939-series brake systems yet.

 

[M35A2-AZ]

You can down load the P2P software here on SS and it will help you trouble shoot the problem.
I used it a lot when I had a brake problem.

 

[Farmallmilitary]

Thanks i found the file now lets just hope i can understand it.

 

[rangereter]

Normally, if your air system drops air quickly when stepping on the brake it means that you have a ruptured diaphram in one of your brake chambers (it could be a service relay valve, but not usually). Of course you have 10 chambers (service) on your truck to choose from. All vent into the intake air tube just before the air filter. If I were you, I would disconnect each white air line (one at a time) at each chamber and have a helper push on the brake to see which diapram/chamber is leaking. If you have a problem with the truck loosing air when the parking brake is released/off, you would normally have a diaphram bad in one of your spring brake chambers (diag. sure, but don't attempt to repair based on your knowledge of this system) which you have four on this truck. Diag the spring brake diaphrams the same as the service (verify which diaphram is leaking based on air leaking from the can with the white vent line removed)... just ensure that the wheels are well blocked to prevent rolling when the p-brake is released. Regards,
Bob

 

[ARYankee]

This is always said a bunch but it really does help....... Read all the tech manuals and printed info on the vehicle. I'm not trying to be an a-hole but it really does help and make a difference. I've learned a lot from reading the manuals when I have some slow time at work or when I'm researching something at home. Also the P2P program will help greatly as well.