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Tractor Maxi knob applies trailer brakes too

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by 04 LowMax, Dec 28, 2023.

  1. Dec 28, 2023 #1
    04 LowMax

    04 LowMax Medium Load Member

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    Feb 3, 2013
    Alberta
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    94 Peterbilt. When you pull knob to set tractor brakes, trailer maxis come on too. Even if you hold trailer knob in. You don't actually have to hold trailer knob in because it doesn't pop anyway. When you unhook for bobtail the red trailer button does its job as it should. I'm thinking something is plumbed wrong to make the trailer maxis come on with the tractor? I've never seen this in over 45 years of driving t&t. Anybody have some ideas? It was a log hauler once upon a time and I wonder if someone modified it so everything came on with 1 button when spun out on a hill.
     
    04 LowMax, Dec 28, 2023
    04 LowMax, Dec 28, 2023
    #1
    Oxbow and '88K100 Thank this.
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  3. Dec 28, 2023 #2
    BoxCarKidd

    BoxCarKidd Road Train Member

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    Aug 26, 2014
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    Bad tractor protection valve? Check air supply from park valves to it.
     
    BoxCarKidd, Dec 28, 2023
    BoxCarKidd, Dec 28, 2023
    #2
    Oxbow and 04 LowMax Thank this.
  4. Dec 30, 2023 #3
    '88K100

    '88K100 Road Train Member

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    Aug 23, 2020
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    I drove ‘87 ford like that,,drove it new off the lot…first time it really annoyed me i woke up and had to get out and beat frozen trailer brakes after a night of drinkin’. After that i would park level ground and drop a wheel chock….only drove the truck for a year,,,,,,another oddity of the truck was it came with a manual window washer pump…that i changed immediately
     
    '88K100, Dec 30, 2023
    '88K100, Dec 30, 2023
    #3
    04 LowMax, MACK E-6 and Oxbow Thank this.
  5. Dec 30, 2023 #4
    Heavyd

    Heavyd Road Train Member

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    Feb 4, 2009
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    Bendix park brake valves are designed that way. Both buttons in, pull out only the yellow, both pop. This a safety regulation.
     
    Heavyd, Dec 30, 2023
    Heavyd, Dec 30, 2023
    #4
  7. Jan 9, 2024 #5
    04 LowMax

    04 LowMax Medium Load Member

    440
    250
    Feb 3, 2013
    Alberta
    0
    Yes. But usually you can hold the trailer knob while you pull the yellow and only get tractor maxis on. Not this one.
     
    04 LowMax, Jan 9, 2024
    04 LowMax, Jan 9, 2024
    #5
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