Parking Brake

Discussion in 'Questions From New Drivers' started by Labrador, Dec 1, 2022.

  1. Labrador

    Labrador Medium Load Member

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    New driver, 3 months on the job. 2007 Peterbilt mixer truck automatic transmission. I was driving down a long approx 3 mile steep road, 15% grade or so about 10 to 20 mph using the STAB method of braking. I pulled off to the side after about half a mile to let traffic behind me pass. I had about 14,000 lbs of leftover sand on board. When I stopped,I pulled the parking brake but left the truck in Drive and the place I stopped had probably again, maybe 15% grade. The parking brake didnot hold the truck (with it in Drive),it rolled forward. On flat ground it easily holds the truck while it is in Drive.

    Anway, was this due to hot brakes? Should this have happened? Do brake adjusters affect the parking brake? I need these air brakes to be as strong as possible as I am often parked on an incline while pouring concrete.Thanks
     
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  3. rockeee

    rockeee Light Load Member

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    Don't know about the new auto's and can't really answer your question but why do you set the brake with it in drive?
     
  4. okiedokie

    okiedokie Road Train Member

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    Don't use the stab braking method. Adj your brakes. Downshift to a lower gear. Engine brake?
     
  5. tscottme

    tscottme Road Train Member

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    Stab braking puts MORE heat into the brakes than holding LIGHT & STEADY brake pressure. It would be best to use jake brake & lower gears and not touch your brakes if they would maintain a safe slow speed downhill.

    Each time you let the speed build, it creates far more energy than maintaining a steady speed. The couple of seconds of no braking doesn't cool the brakes more than the increased speed will heat the brakes. Stab braking is how to overheat your brakes.
     
  6. GreenPete359

    GreenPete359 Road Train Member

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    So one thing to remember is that your parking brakes apply themselves between 20-40psi. So the spring are only applying that much pressure to the brakes, and only the brakes that have a spring brake are activated.

    that said, you may very well only have them on one axle. It’s not uncommon for trucks to be spec’d this way. One axel locked down will not hold the truck on that much of a grade, especially not with it in gear.

    i’m assuming your truck is equipped with an Allison automatic?? If so, then just like your car the transmission is constantly applying power to wheels unless you’re in neutral.

    **Side note some of the Allison’s i’ve driven will automatically kick into neutral when the brake is applied. Not sure if all do, or maybe just newer ones.
     
  7. tscottme

    tscottme Road Train Member

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    Old timers used to use a large enough rock, a wheel chock, or something else to "scotch the wheel" like a wheel chock accomplishes if they have to park on steep grades. Manual transmissions can help when parking on steep grades by shutting the engine off in the appropriate gear to aid the brakes/wheel chock holding the truck.
     
  8. Labrador

    Labrador Medium Load Member

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    Indeed it is an Allison transmission. Unfortunately this one does not shift into neutral when the parking brake is applied, that is a #### good idea I wish it would do that.
     
  9. Ridgeline

    Ridgeline Road Train Member

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    If you can't shift from drive or reverse into neutral with the parking brake applied, it is not road worthy.

    If the parking brakes can not hold the truck, don't drive the thing until the brakes are fixed.
     
  10. RockinChair

    RockinChair Road Train Member

    You should probably keep it in neutral when the parking brake is set, unless you have to keep it in drive to run the equipment (like concrete pumps do).


    Yes. The pushrod pushes on the slack adjuster regardless of whether air pressure (you stepped on the brake pedal) or spring pressure (you set the parking brake) caused the pushrod to extend.
     
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