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

Youre obviously not refreshing and rereading to see if you missed something obvious

 

Even if your tanks hit 120psi if the air bags on the trailer are still inflating it means the trailer needs to finish airing up.

 

Yes, if you listen when you push the button in after you have the wheels where you want them, you can hear the air hissing out.

 

Depends on exact setup but in essence if not exactitude that should be fairly correct

 

Yes but no. More or less thatll get you down the road but the precise part is more as follows


"when there is air in the red line, the air flow CANNOT go to the tandem pins. And when there is no air in the red line, the air flow goes to the tandem pins *provided you have the tandem button activated*."

The technically

 

Correct, as the truck is supplying no air to the trailer, but the tandem button is also an air system, the trailer tank is the only place it can come from