M925 Air Brake losing air, parking brakes wont release

[jwesley74]

I tried searching threads but didn't see any answers to this issue. I'm sure someone can point me in the right direction. Weird issue. Truck is a 1983 M925 ran/drove great. Parked it and an hour later went to drive it and the parking brakes wouldn't release. Even using the Spring Override all I hear is air dumping in intake. If the park brake is on it builds pressure. If I release parking brake I hear it dumping in intake. Checked all glad hands were closed. In fact we used an external source to build air with truck off so we could search for problem. Because it was running driving normal then this happened after being parked leads me to believe its an air release relay or valve that is stuck open. Any suggestions where to look?

Ultimately we had to cage the parking brakes and zip tie the parking brake air switch up so it would stop dumping air. The service brakes worked to drive home.

Anyone have this issue and fix or have the solution would be greatly appreciated.

 

[porkysplace]

I tried searching threads but didn't see any answers to this issue. I'm sure someone can point me in the right direction. Weird issue. Truck is a 1983 M925 ran/drove great. Parked it and an hour later went to drive it and the parking brakes wouldn't release. Even using the Spring Override all I hear is air dumping in intake. If the park brake is on it builds pressure. If I release parking brake I hear it dumping in intake. Checked all glad hands were closed. In fact we used an external source to build air with truck off so we could search for problem. Because it was running driving normal then this happened after being parked leads me to believe its an air release relay or valve that is stuck open. Any suggestions where to look?

Ultimately we had to cage the parking brakes and zip tie the parking brake air switch up so it would stop dumping air. The service brakes worked to drive home.

Anyone have this issue and fix or have the solution would be greatly appreciated.

The troubleshooting section of the Tecnical Manuals .

 

[73m819]

I would guess that a spring brake diaphram is bad, the reason you here the air in the intake is because the vent line dumps there, .

 

[jwesley74]

The troubleshooting section of the Tecnical Manuals .

Thank you, I was hoping for experience on the issue. We troubleshooted for several hours in the field and resorted to caging them to drive it home.

 

[jwesley74]

I would guess that a spring brake diaphram is bad, the reason you here the air in the intake is because the vent line dumps there, .

We checked spring brakes for diaphram leaks. No luck. It appears to be a stuck release valve or relay valve somewhere. We disconnected each spring brake can one at a time. Prior to the issue arising we had been mudding the truck heavily then parked it thereafter. When we started it an hour later brakes would not release. So my 2 cents dirt or crud got into a check valve or release valve and wont allow it to close on the emergency side. Service side holds PSI and works fine. Emergency dumps air ONLY when parking brake is release OR if you push Spring override valve. You can hear air dumping rapidly in the intake. When we engage parking brake, it builds and holds PSI on emergency side.

 

[ichudov]

Hook it up to a compressor from away, so that it is quiet and you can hear where the air is escaping.

 

[jwesley74]

Hook it up to a compressor from away, so that it is quiet and you can hear where the air is escaping.

We did that. It's releasing through intake. I've read the TM on it and we will perform the tests to determine if its the quick release valve. Sounds like that's the culprit under the drivers step. Thanks for the feedback everyone.

 

[rrrr]

keep us posted. I've had my 932 in a couple of mud events and no issues so far.

 

[jwesley74]

Found the problem. It was the double check valve located next to quick release valve under drivers side step. We followed the TM on tests and found no air was in line from quick release to double check. We pulled double check valve apart and found sand/mud had slide plunger stuck in the middle. We cleaned & lubricated and reinstalled. Works normal again.

 

[M35A2-AZ]

Glad you found it. FYI, most of the check valves and quick release valves are off the shelf items.

 

[jwesley74]

Glad you found it. FYI, most of the check valves and quick release valves are off the shelf items.

Thanks, I agree on the off the shelf but the cool thing is they design alot of this stuff to be taken apart in the field. We took the double check valve apart and it had an o-ring so it was easy to clean and put back together. I'm not surprised it had sand/mud in it after what we put this truck thru. Looks like we have to add this to our snorkel vent lines to keep mud out in the future. There's another valve assembly under the step that has one of those floating breather caps on it with an airline that runs from that assembly to the doublecheck. I have a feeling that is the culprit for allowing sand into the line.