M923 Possible Air Brakes Issue

[12BMatt]

Just picked up my M923 from Jeff City MO and drove it back here to Raleigh NC.
However I think I might have Fu*#ed something up. When I started it in Jeff City, the Low Air Alarm would come on of course with the light until it built up pressure. Also the Parking Break light would be on as well until I released it.
Now neither light will come on and the low pressure alarm will not come on. When i pump the breaks there is nothing there until i push in the spring brake, then the pressure "kind of" comes up, but still no lights or alarms.

Info greatly appreciated, like i said worried i fu*#ed something up.

 

[quickfarms]

Are you saying that the truck will not build air pressure now?

 

[12BMatt]

It seems to, when I start it and pump on the brake, it sounds "dry", but when i push in the spring break, then the regualr break it back up and "sounds" as it did before. Does that help.

 

[spartan_185]

There are several one way check valves in your air tank system. I had one stick closed on my 923 a while ago. It was the one that fed the emergency/spring brake system tank under the driver side door. The truck would build air, but the spring brakes would not unlock and the spring brake override would do nothing. Air was not flowing into the brake system due to the stuck valve. The parking brake would not release also. I would go through and check them all. I would bet one of them is stuck due to lack of lubrication.

 

[12BMatt]

One other thing i failed to mention, during the drive from MO to NC, I could hear what i thought was a "pop-off-valve" going off the entire way back. I thought it was "normal".

 

[jw4x4]

The pop-off is normal. The air compressor runs continuously. When the air pressure reaches a certain point (around 120 or so) the valve releases the pressure to keep it in the safe zone. Read up as much as you can on the operation and driving of a large vehicle with air brakes.

 

[12BMatt]

Going over the Tech manual now.

 

[BKubu]

I wonder if the pop-off valve you heard was the air dryer. Often when the air dryer is bad, going, or if the filter in it is dirty, it will go off incessantly. This is not good for the air dryer. Check to see if that could be an issue.

The M939 series trucks have loads of air valves within the air system. Bad or sticky valves are a pain to track down. I am not saying it can't be done, but it involves a lot of trial and error with regard to finding the culprit. These trucks are NOT as novice-mechanic friendly as the deuces or M809 series trucks.

 

[Doug Roberts]

Hey Matt .... did you resolve this issue with your air brakes ? If you did , tell me how it evolved.

Thanks,

Doug