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.
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.
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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
gentleroger and expedite_it Thank this. -
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Removed picture
Last edited: Jun 23, 2021
Reason for edit: Better pic -
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)
Last edited: Jun 23, 2021
gentleroger and expedite_it Thank this. -
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',,
Diesel Dave Thanks this. -
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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? -
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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