Air Brake Issue no air preasure

[lost in the bush]

Hi ..

One of our M923 has an AIr Pressure issue.

• The Rear Brakes are locked because not enough air pressure is available

I drained all four Air Tanks than turned on truck to attempt to build up pressure.
Pressure gauge would not budge off 10lbs. I drained all four tanks again and just like 1st time only tank marked #4 had a little pressure and nothing in the others.



Digging into the situation we found the Air dryer to have an issue so we by passed it.



Next test got air pressure up to 20lb with air in 3 of the four t.anks.


Any Ideas as to where to start looking now?

 

[m16ty]

First thing I'd do is check all the lines for leaks. You could have a pretty large one. The problem is that with a large leak you can never build enough pressure to hear it leaking sometimes.

 

[Flyingvan911]

Check the regulator. Sounds like you aren't getting any air to the tanks.

 

[ntxcop]

This is the same problem I am having with my newly purchased M923. I arrived this morning to retrieve my truck and the air pressure gauges did not budge from 0. I couldn't even get to 10 psi like your truck did.

I had a friend coming in his deuce to pull me home, and decided to cage the brakes. After putting the cage bolts in, I started the truck and noticed (for the first time) that the air pressure started climbing on both gauges. After running the truck for about 5 minutes, I powered it down and checked my cage bolts. All four bolts were loose. I re-tightened them and powered up the truck again. This time the air pressure went to 120 and the warning buzzer turned off.

Due to the late hour, and the fact that the Gov Liquidaiton lot was closing soon, we decided to go ahead and pull the M923 home. I believe there must be some sort of "valve" that is stuck open inside one of the brake chamber assemblies. I have no idea if such a thing exists, but I do know that I was not getting air pressure until I caged the brakes and then the pressure built. I'll have to do some research on it in a few days and will let you know if I find out for sure. If you happen to come up with an answer, please let me know as well.

Thanks
Keith

 

[Bolkbich]

http://www.steelsoldiers.com/5-ton-up/81288-m939-air-system-check-valve.html

 

[gimpyrobb]

You could hook "shop air" to the front glad hand. That should pressurize the syst. and make the leak apparent. Might as well get the rebuild kit for the air dryer too.

 

[Dukeman]

Sorry, what is "caging" the brakes and what does it do?

 

[ntxcop]

Sorry, what is "caging" the brakes and what does it do?

Caging the brakes is the way you "unlock" the brake by manually retracting the springs that cause the brakes to clamp down.

On each of the 4 brake carriage assemblies, there is a "tool" which is a bolt with a "T" on one end. It is stored in a "sleeve or shaft" on the brake carriage assembly. You unscrew the bolt and remove it from the shaft that it's stored in. Then you pop off the plastic cover on the end of the brake carriage assembly and insert the "T" end of the bolt into the carriage assembly. Once the bolt is inserted into the carriage assembly, you put the washer and nut on the bolt and screw it down. As you screw down the nut, the "T" end of the bolt pulls on the spring and unlocks the brakes.

I've added two pics since I am not a diesel mechanic, and may be using names for parts that are incorrect. You can see the empty "hole" on the carriage assembly where the tool is stored.

This is how you unlock the brakes without air pressure to tow the truck. Like I said, I'm not a mechanic, but the diesel mechanic on site was puzzled too. Once I caged the brakes, my air pressure started to build, so I think for my problem, there is a leak in one of the carriage assemblies or a spring valve that was stuck. My truck had been sitting in a lot for about a year.

Hope this helps.