Air Brake Scare

[joshsp]

Guys i need some help. I had a bit of a scare a few days ago when my 5ton(M923A2) shut off at the top of a hill due to the fuel shutoff solenoid failing. In short when the truck died i pushed the brake pedal to the floor and it didnt hold. I began to roll backwards until i could get the wheel turned enough to put my self in the ditch which stopped the truck. Thank god i was on a back country gravel road without anyone behind me.

Any idea why the truck didnt stop when i ran out of air? It doesn't take long when the truck is off. I did end up tying back the fuel shutoff did get the truck started and did make it home but i am now nervous to drive it after my close call.

Thanks for your help.

Josh

 

[M35A2-AZ]

Have you checked the brake shoes, adjusted the brakes? Does the parking brake hold the truck?
Sounds like some of the service brakes are not working.

 

[sandcobra164]

These trucks don't automatically engage the parking brake when the primary air system bleeds off. If you had pulled up your parking brake lever, you would have stopped. Newer trucks that require you to push in the control on the dash will set the parking brake if primary air drops below around 60 psi.

 

[Trailboss]

Are some or all of the brake cans caged on the rear axles? If so, the truck will still roll freely when you have no air pressure instead of locking the rear brakes.

 

[joshsp]

Thank you all for your response. I have not yet checked the shoes. I have never done it before on somthing with air brakes and still need to do a bit of reading. If you have any pointers or point me in the right direction in the manuals that would be helpful. The parking brake did not hold the truck when i picked it up a few months ago but with a turn or so on the handle it hold fine now. Wish i would have thought of pulling it just was freaking out about rolling all the way down a big hill.

I don't think any are caged. I will have to look when i get home. I did have a full safety inspection completed on the truck when i first picked it up. I sure hope if it was they would have said somthing.

 

[M35A2-AZ]

You need to check to see if any of the spring brakes are caged and get the spring brakes (parking brakes) working before you drive your truck again. The brakes (spring brakes) should come on when you get low on air, like below 60 psi.

 

[CUCVFAN]

I'm glad no one was hurt. That could have been bad. Is the spring brake override on the dash pushed in?

 

[M35A2-AZ]

I'm glad no one was hurt. That could have been bad. Is the spring brake override on the dash pushed in?

That is a good idea to check! If it is pushed in the dash light should be on.

 

[98G]

One good mash isn't enough to deplete the air. My guess says that the service brakes are out of adjustment and the springbrakes are overridden.

 

[joshsp]

It should be out unless it some how or someone was pushed it in. Something i will definitely look at when i am back home from work. Its my understanding that it is always out. Correct? Unless you need to move it before letting the air tank fill.

If it was pushed in that may explain why i couldn't stop as i was loosing air quickly with my foot on the brake and motor off?

 

[98G]

You weren't losing air that fast unless you have major leaks in the system.

 

[M35A2-AZ]

Yes, always out.

But if the truck has no air the spring brakes will come on no matter ware the override button is.

 

[98G]

I sent you a PM with my number. Call me when you're at the truck and I'll walk you through some things...

 

[Coffey1]

It's possible you lost the Q2 valve for it to dump air that quick. Did the brakes work and hold air on the drive home.

 

[Jakelc15]

View attachment PI221.pdf

This will make you think twice about driving on poorly adjusted brakes.
It also shows how to properly adjust most

 

[joshsp]

The truck drives and stops fine. You actually have to be gentle with the brake pedal or you will eat the steering wheel if you apply to much pressure.

 

[simp5782]

Just cause you turned the nut on the parking brake cable does not mean it's functioning properly. Your e brake handle does two jobs. If it wasn't holding when you first bought the truck then your valve that dumps to the spring brakes is bad even with no parking brake cable attached if the handle is up the truck should not move.

Gravity will only over power a proper operating 939 brake system if you are severely heavy at 50k + and only a steep grade
You are losing air after shut down prob means you have a spring brake diaphragm leaking. Especially if your release valve isnt working

Sent from my SM-G935P using Tapatalk

 

[simp5782]

The truck drives and stops fine. You actually have to be gentle with the brake pedal or you will eat the steering wheel if you apply to much pressure.

That could also be a sign of an air line or two not hooked up properly

Sent from my SM-G935P using Tapatalk

 

[74M35A2]

Yes, always out.

But if the truck has no air the spring brakes will come on no matter ware the override button is.

My understanding of the spring brake over-ride button is that once pushed in with air above 60psi, the spring brakes will not work/grab once the air drops below 60psi, or when the parking brake is set. It must lock air to them. But, I may not have that correct. Should read the manual or just try it on a slight hill once I get home. Sorry, slight derail to the OP's deal.

 

[M35A2-AZ]

I think you may be right, but if the air system goes below 60 psi the spring brakes should come on.
For the most part I never use the over ride.