Losing air pressure when emergency brake is on.

[mdainsd]

Hello forum. I just picked up a M931A1. I spent the day working my way through the Air Brake system. Ive had the double checks out and inspected them, also the hand brake air valve.

Ive gotten the truck to where once its started and idling, pressure rises in primary and secondary system at about the same rate, IF the hand brake is on. Turn off the truck and the air bleeds down slowly, but still faster than I would like. After thirty minutes both gauges are down to about 60lbs.

BUT, If I lower the hand brake control to driving position the pressure in both tanks starts to immediately drop. it will drop down to 60lbs in a little over a minute with the engine running. With the engine off it goes all the way to zero.

I tried this: Ran the truck until both tanks were full ~125lbs. Shut if off so I could listen for air. Then I had a friend in the cab release the handbrake while I went around listening. Still hard to tell but it seems both spring brake chambers on the middle axle have air coming out of them with the hand brake released.

Any ideas?

Thanks, Mark

 

[juanprado]

Spring brake chambers have 2 rubber pancakes on each side. Chambers need to be replaced or rebuilt. Do not attempt rebuild unless you know exactly what you are doing and have done air brakes like this before. Spring in chamber under pressure!

Probably need to do all of them. Do the brakes release and is tractor drive able? The tractors are notorious for water leaking into the drums and the brake shoes rusting to drums and locking them up.

 

[mdainsd]

It drives, but I won't do it anymore, LOL. The guy I bought it from disclosed that there was a problem. It was delivered in place with the spring brakes caged. He had me drive it around an open field, and I did so slowly, no faster than the mechanical part of the parking could could stop me.

I haven't taken apart these brakes before but have read the cautions of the high spring pressure inside.

Where does one hunt down the rebuild kits if there is such a thing.

And, thank you!

 

[simp5782]

It drives, but I won't do it anymore, LOL. The guy I bought it from disclosed that there was a problem. It was delivered in place with the spring brakes caged. He had me drive it around an open field, and I did so slowly, no faster than the mechanical part of the parking could could stop me.

I haven't taken apart these brakes before but have read the cautions of the high spring pressure inside.

Where does one hunt down the rebuild kits if there is such a thing.

And, thank you!

Mostly every part for the truck is listed here and available thru your local napa. You need to replace your diaphragms first and go from there. They sit and get weak. Atleast your Spring can diaphragm and service on that side. You would need 4 of the DP16 and 10 of the DP12s to do all the truck.

There are videos on youtube for M939 trucks especially for spring brakes under "NELSONSTUDIOS".

Easy way to tell which spring can isn't working properly. Chock the wheels. Get your brake caging tool. Get the truck to max air pressure. Release the emergency brake. See which spring brake is pulled back all the way in the canister by looking thru the caging port. When the brakes are released you should be able to install caging bolt and tighten it 3 or 4 turns and it will be all the way released. If not then you know that one has an issue.

https://docs.google.com/spreadsheet...Bn7VomUUJDuk_aG_to5pzHqzkBc/edit?usp=drivesdk

 

[juanprado]

It drives, but I won't do it anymore, LOL. The guy I bought it from disclosed that there was a problem. It was delivered in place with the spring brakes caged. He had me drive it around an open field, and I did so slowly, no faster than the mechanical part of the parking could could stop me.

I haven't taken apart these brakes before but have read the cautions of the high spring pressure inside.

Where does one hunt down the rebuild kits if there is such a thing.

And, thank you!

Haldex makes them. Napa or any HD parts store. They are sold by # to indicate the size.

Check this thread
https://www.steelsoldiers.com/showthread.php?132401-M939-brake-chamber-diaphragm-problems

find thread and you tube video by acme66

 

[mdainsd]

Thanks Gents! A pleasure.

Mark

 

[fasttruck]

Sounds like you need 4ea dual service brake chambers. Trying to repair parking brake side is usually not a good idea.

 

[98G]

Do not attempt rebuild unless you know exactly what you are doing and have done air brakes like this before. Spring in chamber under pressure!

p.

This bears emphasis. There is a lethal amount of energy stored in that spring. It will literally kill you dead if mishandled.

I strongly suggest that the first time you work on them you have someone with you that's done it before.

 

[74M35A2]

There is no such thing as emergency brake.

 

[98G]

There is no such thing as emergency brake.

when the air system fails and the air leaks out and the springbrakes apply, that's the emergency brake. As separate from the park brake and the service circuits.

 

[mdainsd]

This bears emphasis. There is a lethal amount of energy stored in that spring. It will literally kill you dead if mishandled.

I strongly suggest that the first time you work on them you have someone with you that's done it before.

I agree whole-heartedly.

I have a friend who does this stuff all the time. I'll trade him dinner out or something.

 

[BEASTMASTER]

those things are deadly. I know a guy that lost his lower arm taking one apart, and he was a professional truck mechanic. the caging bolt broke and that can was like a bomb under that truck. I have done quite a few in my early years and was scared until I got the thing back together. I don't even think the truck shops bother with them. just buy all new cans and live a longer life.
been there, done that, and lucky to still be here. " REALLY"

 

[rbrown225]

Also a way to track air leaks is to make up a hose that connects from the left front glad hand to an air hose hooked up to an air compressor. You can control the air with the valve at the glad hand and be able to hear the air leak to track it down.

 

[mdainsd]

Thanks for the tip. I will rig up a hose to do just that! I picked up new diaphragms at NAPA and have a failsafe on the way from a SS member.

 

[74M35A2]

On the can rebuilds, I have heard many say exactly the same thing. It sounds like you just need to read instructions carefully, and follow what it says. My dad did one on his backhoe, and it went flawless, he has never done one before. I think if done correctly, they just easily come apart like anything else? Can one clamp the entire can in a bench vice or C clamp to lessen the chance of injury in case proper procedure is not followed? I have never serviced one, so apology for my ignorant question, but it will likely need to occur somewhere in my future. Really looking for what is the missed step that causes injury, thanks.

 

[Floridianson]

I don't over tighten the nut/cage bolt. I inspect the ears on the cage bolt are in good shape. I make sure the cage bolt is locked and hold / twist the end of the bolt till the nut is run up and there is pressure on it so it stays locked. I treat the end cap with the spring compressed like the cage bolt could let loose and never get any part of my body in the line of fire. I clean the clamp and housing of all paint and dirt so the clamp seats correct.

 

[mdainsd]

Im actually looking forward to tackling this job. I design and work with very high pressure equipment (20Kpsi) at work. I have the safety side drilled into me. Ive been reading all I can on this job. The two things that are starting to stand out are 1) the condition and quality of the caging bolt, 2) the torque to be applied to the nut on the caging bolt to ensure spring compression without over-tightening.

Needless to repeat but - DONT PUT YOURSELF IN THE FIRING LINE, think about where this thing will go if something goes wrong.

 

[Floridianson]

On the tightening of the nut if you ran it down with a small handle wrench and it got super tight you went to far.
Your right mdainsd it can only go two ways if the cage bolt comes loose so hold it accordingly and keep children and love ones away from work area. Also it's a good idea to put some oil on the threads of the cage bolt. When installing the cage bolt make sure it goes 1/4 turn clockwise to the lock position.

 

[mdainsd]

All done! with mixed results.

I did get it down to a science, the last two took me about 15 minutes each. Builds pressure (as always) release the hand brake and the pressure drops a bit but the compressor keeps up and brings it back to cut off, then the cycle repeats, down to around 100psi, compressor kicks in charges back up. Drove it around for a while and both pressure gauges stayed between 100 and 125 psi. SO, unfortunately I have not found all the gremlins yet. Going to get the truck inside and hook it up to shop air and leak hunt.

The things I found: one badly deteriorated spring brake pancake, age cracks in the other three. Water has been in the brake chambers, so it must have been everywhere... Its a Marine truck so no air dryer, what were they thinking? I hooked up the failsafe I bought here and the alarm never went off even though air was where it should be, so I disconnected it before it deafened me.. On the drive home I got to thinking. Was the alarm going off because the hand brake was set? After I get cleaned up its off into the operators manual for answers.