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. expedite_it

    expedite_it Medium Load Member

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    I work for a trucking company in which almost all of our trailers have the button that you pull out to make the tandem pin go in, not the old style of trailer with the big handles that one pulls to mechanically make the tandem pins go in.

    First, I called breakdown at my company, and breakdown instructed me to idle the truck until the air pressure in both brake systems reached at least 120 psi. Then to pull the red valve. Then pull the tandem pin button towards the rear of the trailer. Then to pull the truck forward to slide the tandems. But the tandems would not slide. The truck just dragged the entire trailer forward without the tandems spinning.

    For the last several trailers in which I had to slide the tandems, I had a lot of trouble sliding the tandems. Sometimes the tandem pins would pop out prematurely, even though I did not push the tandem pin button in. Last night I could not slide the tandems to the rear at a shipper, and my company had to hire a mobile mechanic to fix this. When the mobile mechanic first had me attempt to slide the tandems, he instructed me to idle the truck until the air pressure in both brake systems (primary and secondary) went up to 120+ psi. Then he had me pull the red valve (trailer air supply knob).

    The mobile mechanic discovered that I have a leaking valve somewhere in my emergency air system when he found out that air came out of the glad hand of the emergency line when I pressed the brake pedal. The mobile mechanic said that no air should come out of the glad hand of the emergency line when I pressed the brake pedal. When I tried to slide the tandems, I was pressing the brake pedal when I slide the tandems so the truck would not roll prematurely. Apparently, the air that came out of the emergency line glad hand was making the pins pop out premature, making it so that the tandems did not slide. The mobile mechanic was able to help me slide the tandems by disconnecting the emergency line glad before I moved the truck forward to slide the tandems to the rear. This technique worked.

    We have to have the red valve pulled out to get the tandem pins in when we pull the tandem pin button at the rear of the trailer. Therefore, it seems to me that to get the tandem pins in, there has to be no little or no air in the brake lines in the trailer. When I first started the process of sliding the tandems, both breakdown and the mobile mechanic instructed me to idle the truck until the air pressure in both brake systems reached 120 psi. However, in my opinion, there seems to be a contradiction here. If there must be little or no air in the brake lines in the trailer in order to get the tandem pins in, why did I have to build up the air pressure in both air systems to 120+ psi before I pulled the red valve out and then pull the tandem pin button?
     
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  3. truckdriver31

    truckdriver31 Road Train Member

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    lot of things on truck and trailer that need air. theres only so much your tanks and air compressor can do.
     
  4. skallagrime

    skallagrime Road Train Member

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    A few reasons, 120 is probably not necessary.

    Lets begin at what happens when you push/pull the button on the tandem pin. Thats an air activated manual system, so to get the pins in, it will need air in the trailer tank. If the system has a leak in it, being at full pressure gives you a longer period of time to futz with it

    Your tractor being at full pressure is so you can release your brakes and move forward or backward hitting the brakes a bunch before running the compressor. Again 120, not necessary, but best practice.

    Now the mechanic is correct, pushing the brake pedal should never result in air coming through your red line. The pedal or trolley valve should only push air to the blue line.

    Your trailers will not pull the pins in if there is pressure on the red line, thus activating the pedal when the malfunction was there popped the pins back in the locked position.

    There is a check valve allowing the trailer tank to have air in it but not supply it to release brakes when there is no air pressure in the red line.
     
  5. expedite_it

    expedite_it Medium Load Member

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    Excellent post, skallagrime. But i still don't understand this 100%.

    So to get the tandem pins in, there must be air in the trailer air tank? Then when i pull the tandem pin button to get the tandem pins in, why must the red valve be pulled out? I would assume pulling the red valve out causes no more air to be supplied to the trailer air tank.

    Are you saying that the reason people say my tractor must be at 120 psi is to ensure that the air pressure is not so low that the spring brakes on the tractor are activated?

    I wonder why my trailer will not pull the pins if there is pressure on the red line. That might be something that would be difficult to teach just through writing posts though.

    Is that check valve on my tractor?

    That check valve is the defect that was causing me to be unable to slide the tandems when i had the glad hand connected to trailer; correct?
     
  6. MLC Adventures

    MLC Adventures Light Load Member

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    There's no defect. It's a safety so that of your brakes are released your slider automatically locks. You shouldn't be driving with slider pins out.

    A trailer may sit for a month, and it runs out of air. You often need to release the brakes to build air in the trailer, then set the brakes to slide. Otherwise, you won't have enough air to operate the slider.
     
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  7. bad-luck

    bad-luck Road Train Member

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    There are springs that keep the pins lock on your trailer. The air unlocks the pins. For future reference once the pins are unlocked, you can use a small pair of vice grips to keep the Button out.
     
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  8. expedite_it

    expedite_it Medium Load Member

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    When the pins are "locked", are the pins to sticking out, or are the pins in when they are locked?

    Just out of curiosity, how do you know that there are springs that keep the pins locked on a trailer? Did you used to be a trailer mechanic or what?
     
  9. bad-luck

    bad-luck Road Train Member

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    Locked they are sticking out, and you can't move your trailer wheels. Not a mechanic just been trucking all of my adult life. Just look under the trailer.
     
  10. fishonron

    fishonron Medium Load Member

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    Sometimes when the pins won't retract all the way is because they're bound up and rocking the trailer back and forth will free them.
     
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  11. Six9GS

    Six9GS Heavy Load Member

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    I commonly drive the pneumatic tandem pin trailers like you describe. Back on the tandem assembly, there are a couple of air tanks. One of them stores air used for the trailer brakes and another store air used to pull in and push out the tandem pins. Chances are when you first hook up to a trailer, those tanks are empty. When you push the trailer brake knob in (the red knob in your cab), air starts to flow into the trailer (through your red gladhand). It can take a few minutes to 'charge' those couple of air tanks back in the trailer tandems. I can hear the air hissing in the valve assembly in the cab and I wait until the hissing stops before I try to slide my tandems. It can take 3 to 5 minutes. Until that tandem pins tank is charged with air, it may not have enough air pressure available to suck in the tandem pins. That's why you habe to wait a few minutes.
    Anyway, once those air tanks are charged, you can release the trailer brakes (push the red knob in) and then the tandem pins will have enough available air pressure to suck in the pins when you push (or pull, depending on the specific style) in the knob on the tandem assembly to adjust them. Sometimes one or more of the pins may be sorta jammed between the rail and the tandems so that they won't pull in. I have a hammer I take with me. Once I push in the knob on the tandems, I look to see that all 4 tandem pins actually get sucked in. If one gets stuck, I'll hit it with my hammer and that will usually un-jams it enough for it to pop in.
    Important to know too is that once you push in the trailer brakes (red knob in the cab) it will automatically pop out the tandem pins. You can also pull (or push) the knob on the tandems to pop them out. But, it is done automatically when you push in the trailer brake release knob (red knob) in the cab.
    I hope this helps explain how they work and how you can work with them to adjust your tandems.
     
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