M931a2 not building air unless brake pedal applied

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Redneck92

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Hey guys, I parked my Truck about an hour ago and checked the trailer connector wiring.

Friend of mine sat behind the wheel while I was measuring pins at the connector.

I asked him to step on the brakes and he pushed hard in the pedal, released it and a very loud goose sound appeared. Louder than normal. I did not think longer about that at this point.

Testing was finished, wantet to drive the truck back in the shop when I noticed, that it was no longer producing air pressure.

I touched the delivery line of the compressor and it was getting hot, so compressor is working.

Then I noticed, that when im pressing on the break pedal, the truck is producing air pressure.

As soon as I release the pedal, both primary and secondary gauges immediately drop to zero.


Could it be that the brake pedal went when my friend pushed to hard?

Where should I start to look?
Until this day the truck ran great, braked great, build pressure up quick.

Hope someone can help.

Greetings Mike

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Redneck92

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So I tried again this morning. Headed in the cab, noticed that the park brake lever was down. Pulled it up.
I startet the truck up, and went outside to check all the valves on the emergency and service glad hands, let the truck idle, walked around it to hear for leaks. I suddenly was scared from a pop off sound, went in the cab and what do you know....

Truck aired up just like normal... Can't believe it xD

So I went ahead, shut it down and drained the air tanks completely.

Startet it up and let it idle.

Pressure build to 60 psi in 1 minute 45 seconds and took a total of 3 minutes 50 seconds to reach 120 psi. Everything is back to normal.


Could it be possible, that a valve was stuck or something? I can't really find a explanation for what was happening here.

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Last edited:

simp5782

I'm the dude, man!
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Yes the treadle valve was bypassing cause he hit the pedal really hard with the spring brakes applied.

Open the air line at the rear of the wet tank and add marvels mystery oil and at a few other points to lube the valves and what not.
 
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