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

That is correct

 

I probably learned the same way that @skallagrime learned - I had a problem with my tandems and only my wits and some basic tools to work with. A little trial and error goes a long way to achieve comprehension. Play around with things, see which lines connect with which doodads.

On a side note - looking at post times I get the sense that you were reacting to posts instead of reading them. When you've gotten several similar answers to your first question, but you still don't understand, before asking the second question take a couple minutes to think things thru, then ask the follow up. When you slow down and think, things tend to become less mystical.

 

Nope, started with spread axle flatbed, still doing that 7 years later, but bought my own truck after 1.5 years with much less money than is wise and at that point you either learn to diagnose and fix stuff or you file bankruptcy. Ive also always just had an interest in the way things work and the stubborn streak to buy tools and just try instead of giving up or paying someone else. It helps to be smarter than the average bear but just tinkering will give you practical hands on knowledge that trumps most certifications

 

Youre right per the physics but wrong because you dont know how the physical part works.

Your 5th wheel slide is the easiest thing to examine in this regard, you tap a button or flip a switch and air retracts the pins from the 5th wheel rails. It doesnt do this by vaccum "sucking" the pins in, it does it by activating a pneumatic piston that is hinged and will return to a resting state (normally by spring action)

See pic, thats my 5th wheel slide, and cylinder. Normally closed as you see it, but button in cab strokes it out

 

And now to confuse you, some shops use pressurized air to create a vacuum for say checking your coolant system for leaks before refilling it or drawing the vacuum on your ac system via the venturi effect