When sliding tandems, why does air pressure have to be at 120+ before pulling red valve?

Discussion in 'Freightliner Forum' started by expedite_it, Jun 21, 2021.

  1. scottied67

    scottied67 Road Train Member

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    It takes air pressure to actuate the release valve. When you get the button pulled out, the first thing you should do is put the tractor in reverse to relieve any pressure off the pins and the slider assembly; after all, you have been pulling the trailer all day and the pins are pressed hard against the slider assembly. This little trick was even lost on my mentor over 11 years ago when we had the pull handles for releasing the tandems, I would put the tractor in reverse first thing and he'd be going ape back there, gesturing that I was going the wrong way lol.
     
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  3. skallagrime

    skallagrime Road Train Member

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    If i remember in the morning ill try and draw the diagrams illustrating my post on the subject, its much easier to explain in person and if you dont already have a pretty good grasp of pneumatic, electrical or hydraulic systems, even diagrams wont help that much. Luckily for us, pneumatic is the easiest of those three
     
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  4. expedite_it

    expedite_it Road Train Member

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    I would like to see your diagrams.
     
  5. skallagrime

    skallagrime Road Train Member

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    Removed picture
     
    Last edited: Jun 23, 2021
    Reason for edit: Better pic
  6. skallagrime

    skallagrime Road Train Member

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    Day late n buck short but here it is. Brightened pic a bit.

    Of note is that there should be a 3rd sketch bottom right (C), illustrating when you arent trying to push the trailer pin button, AND no red air coming in no air goes anywhere in that valve (only source would be air tank)

    20210623_081249.jpg
     
    Last edited: Jun 23, 2021
    gentleroger and expedite_it Thank this.
  7. 201

    201 Road Train Member

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    Oh, how we've changed, "air pins", HA! You don't know how good you've got it. It's easy to forget, you have a lot of weight on those pins, and years ago, sometimes it took a mighty whallop to release them, so it's going to take a lot of air AND possibly a whallop with a hammer. Again, I don't mean anything against the driver here, at least they are asking, but something as simple as adjusting trailer tandems, and not knowing the operation, is crucial to driving a truck, and how this could have been overlooked in any kind of schooling, gives me the creeps. I've seen more than one wagon sitting on the frame with rear wheels all askew. Just sayin',,
     
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  8. expedite_it

    expedite_it Road Train Member

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    My tractor being at full pressure is so I can release what brakes, the air brakes (by pushing the brake pedal) or so I can release the brakes by pulling the red valve, or is it so I can release the brakes by pulling the yellow valve?
     
  9. expedite_it

    expedite_it Road Train Member

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    Everyone keeps talking about this in terms "releasing the brakes". I don't know exactly what that means. I think "release the brakes" means to take one's foot off the brake pedal, but I don't know for sure.
     
  10. expedite_it

    expedite_it Road Train Member

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    To me, there seems to be a contradiction in the two parts of what you wrote that I put in boldfaced, red font. First, you say that when I push the red valve in, air starts to flow into the trailer (through the red gladhand). Then you say that once the air tanks are charged, I can push the red knob in and then the tandems will have enough available air pressure to suck in the pins when I push (or pull) in the knob on the tandem assembly to adjust them.

    Let's say I just hooked up to a trailer, and I want to slide the tandems. First, I push the red valve in to make air flow into the trailer tanks. Then, by your instructions, I am supposed to PUSH THE RED KNOB in so that the tandems will have enough available air pressure to suck in the pins. But by your instructions, the red valve is ALREADY pushed in. So why would you say to push the red knob in when the red knob was already pushed in in the beginning so that I could get air into the air tanks?
     
  11. expedite_it

    expedite_it Road Train Member

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    I studied your diagram for a few minutes today, and I still don't fully understand it. I believe I have to go pick up a load now, and I won't be able to study it until after my shift. Hopefully, I will be able to understand it later. Thanks for the effort.

    On your #2 diagram, are you saying that if there is air in the red line and the tandem button is pushed, the air flow will ONLY go towards the air tank (as opposed to the airflow going to the tandem pins)? Are you saying that if there is no air in the red line and the tandem putton is pushed, then air will flow to the tandem pins? On the "A" part of your #2 diagram, I am assuming that you have just hooked up to a trailer, and nobody has touched the tandem button at all yet.

    By the way, when I am trying to understand this, I am trying to understand how this works on trailers in which you PULL the tandem button to get the tandem pins in, and you PUSH the tandem button to get the tandem pins to stick out.
     
    Last edited: Jun 24, 2021
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