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

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

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.