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

You're over thinking.

That is the linkage for your pin. Air is used to retract that linkage and pull the pin in. It's not pulling a vaccum, it's simply using air to make the pin retract.

If the parking brakes are released, air is immediately evacuated from the pin system, forcing the pins to relock.

 

One of the Car Talk puzzlers was about a guy who noticed he forgot to put a washer in the oil drain plug after refilling the oil and rigged a vaccum to keep the oil from draining out so he could replace the plug.

 

As he said, youre overthinking it, a piston goes up and down, but somehow creates rotational force. Stop getting hung up on the full physics of how it works, especially if you dont take the time to crawl under your trailer and snap some pictures to see how its set up

 

When you said sucking them in, I just thought that was your explanation it does not suck anything in period it is actually an air piston. Crawl under the trailer during the daytime use your flashlight and you will be able to see how it is set up. Is there actually on some buttons an actual tube that comes off the exhaust port when you push the button in You can feel the air coming out

 

To give you an idea just how it works. Take a look at your air slide 5th wheel it works the same way

 

Yes, the linkage is pushed in, just like on a mechanical mechanisim.

 

Overthinking at it's extreme.

I'm sure that everyone here has had issues with the tandems not wanting to slide from time to time.

I gave my best answer in post #9. Once I figured that out I was good to go 90% of the time.

The other 10% when I know the pins are released and the sliding is rough I just squirt some WD in each of those rail holes and good to go, especially if the WD had a few hours to do it's thing before I had to slide them again.

What's going to happen when your ABS trailer light starts shining? Or god forbid the Check Engine Light.

I do give kudos to the members here for trying to explain how the system works but if driving is not for you and being a mechanical engineer is in your blood then there's a school for that.

 

Its simple really... When air is given to the trailer through the red air line (red knob on dash pushed in) the air tanks on the trailer fill with air, this is called Charging the Trailer. When the red knob is pulled out the lines leading to the tanks loses pressure but the tanks remain charged. This air in the tanks is what the trailer uses to retract the pins so the slider can move, but there is a safety interlock valve in the lines leading to the tanks. If that valve senses pressure in the red air line it wont let the pins retract. So if something was leaking causing pressure to cross from the blue lines into the red lines that safety valve would sense the pressure and lock the pins back into the out position, therefore not able to slide.

Basically, AIR PRESSURE retracts the pins to allow sliding. The higher the air pressure the faster and easier the pins retract. SPRING PRESSURE pushes pins out. A safety valve over rides the pin release button if there is air pressure in the red line.

I'm sorry if this next bit sounds a bit harsh... If you cant understand what it means to "Release the brakes" how did you manage to pass the AIR BRAKE portion of your CDL test?

When you push the yellow knob in on the dash it releases the tractor brakes (the tractor will roll easily). When you push the red knob in on the dash it releases the trailer brakes (the trailer will roll easily). When you pull those knobs out it sets the brakes. The foot pedal works the same way, pressed and it sets the brakes, lift foot off pedal and it releases brakes.