Air loss through dash valves when trailer is loaded

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Pete_379, Jan 18, 2020.

  1. Pete_379

    Pete_379 Bobtail Member

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    May 16, 2018
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    When my fb trailer is loaded and on uneven ground, I get air back feeding through the parking brake valve in the dash. You hear no air when on level ground or at highway speeds. Air pressure will drop down to 75 psi depending on how far into the rough yard I have to travel. When empty I dont notice any air in the cab like when loaded. The trailer has hendrickson suspension and is a spread axle. Does anyone know of what could cause this?? TIA
     
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  2. Dave1837

    Dave1837 Light Load Member

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    Depending on the compressor it shouldn't fall below 100 PSI, ever, unless there's an air leak somewhere in the trailer. You'll hear air flowing through your protection valve while the compressor is running and you'll hear air coming out of your air suspension when on uneven ground. That comes from your air leveler on your air suspension. My compressor runs a lot when I haul flatbeds, or any loaded air ride trailer too
     
  3. Pete_379

    Pete_379 Bobtail Member

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    May 16, 2018
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    It's weird though I see the trailer lean and that's when the air comes into the cab at the brake valves. I also see air exhaust from under the trailer when it shifts but this is the only trailer that does this on my truck. It seems like its pushing air back through the truck when the suspension moves alot.
     
  4. AModelCat

    AModelCat Road Train Member

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    Is it leaking when the trailer brakes are released? If so I don't see it being the dash valve. The exhaust port on that valve will be closed any time the knob is pushed in. Can you confirm 100% its the park control valve that the air is coming from? Because I truely have doubts its that valve that the air is coming from.
     
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  5. baha

    baha Road Train Member

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    You may be hearing air passing thru the Red valve because the air ride leveling valve is leaking under the trailer when its loaded, when its leaking pull the yellow truck emer. brake out and leave red button in chalk wheels and ck leveling valve for a air leak under trailer, it may be leaking out the vent ?
     
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  6. Dave1837

    Dave1837 Light Load Member

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    Nov 16, 2019
    Pennsylvania
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    I'm not sure what that could be, sounds like a leveling problem with that specific trailer
     
    baha and Pete_379 Thank this.
  7. Wvcoalpacker

    Wvcoalpacker Bobtail Member

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    Dec 22, 2017
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    id Say your leveling valve is bad. It’s pushing air back through the suspension switch.
     
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