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

My Bad! Once you've sat there and pressurized the tanks, you can PULL the red knob to adjust tandems. Sorry for the confusion this created for you!

 

Releasing the brakes means pushing in the red knob. This supplies air to the trailer and overcomes the spring brake tension that automatically applies the brakes when no air pressure is present. It kinda seems backwards, but most of us learned about this when getting our CDL. Mechanical spring brakes will apply the brakes when air pressure is removed from the trailer. This is to hopefully prevent the trailer from going anywhere when it's not hooked up to a tractor. To overcome these mechanical spring brakes, you supply air to the trailer and the air pressure will overcome the springs to release the brakes so the trailer tires can roll.

 

It's a context thing. In the context of getting scaled, release the brake pedal (the red knob remains pushed in). In the context of sliding tandems, releasing the brakes means pushing the red knob in.

 

I am unconcerned with if the tandem button is pushed or pulled in order to release the pins in the tandems, and there are a variety of ways the setup could be for the mechanics of how that works, what i was shooting for is you understanding why air in your red line caused you so much difficulty.

Part a is essentially the trailer ready to roll down the road
Part b is trying to move the tandems

Ignore the details of the rest of the stuff youre getting caught up in, exact terminology push/pull doesnt matter so much

 

You also do not need to know that

What you need is to be able to see if the pins on the rail have gone in and are no longer holding the tandems in place or not.

Procedure, hooked up to trailer fully ready to roll down the road, set all brakes, go push or pull pin, get back in cab, release tractor brakes rock it a little, walk back to tandems and see the pins are in, get in truck, slude forward or backward. Release trailer brakes, rock trailer back or forward with trolley arm pulled (brakes to trailer only (or re-set trailer brakes)) so the pins settle into position.

Go back to tandems and inspect to make sure pins are locked in place.