What is the Johnny bar for?

They don't teach it that way anymore.
Now all of that is done with the parking brake or service brake.
In fact, the only question about the trolley valve on the written test was asking what it's function was.
Never told to use it or when. (Indiana)

I've also heard it called the oh #### stick because some drivers will grab it and mash the hammer if they feel a jackknife coming.

Edit: sometimes I use it when starting up hills and I'm too lazy to keep my foot on the brake that firm

 

Don't you need it to slide tandem? Noob here soon will be going to school

 

It saves you from a jacknife in a less-than-ideal traction situation (I know from first-hand experience).

Among the other uses mentioned by other uses.

And I've heard some tanker drivers use it to prevent sloshing in the trailer. Oh yes, and some more "Old School" truckers refer to it as the "company brakes".

 

After a long day i use it atvstoplights instead of the foot valve.

 

I swear if normal people got on here and read some of the stuff we do their jaws would drop.
I can hear em now, "Don't they just sit there and drive all day? Why is holding the clutch and brake hard after 700 miles? Truckers are lazy"

 

I worked sideloaders for pepsi and budweiser, the hand valve was a well needed break after13 hours lumping fizzy sugar water in 100 degree heat.

 

If you own the truck you're driving and doing power only (meaning you're pulling a company's trailers with your own truck), you use it to slow down and save your truck's brakes

A lot of guys who do off highway logging will use it to keep things straight when coming down the mountain.

 

LOL!!!
Its also good for starting fishtails and powerslides..I run a lot of freshly bladed skwirly dirt roads, so Im driftin alot on purpose when Im empty..

Yesterday i learned you need to kill the traction control first, today I learned you need to kill the abs also!! Geez these new trucks, gotta push 17 buttons just to slide around a bend !!!

Get to know the truck!!! Open wet roads with no one around can be fun too...Nail them brakes see what they do!!
Ive been very surprised each time Ive done it...Sometimes I had way more brake than i expected, sometimes i had way LESS brake than i expected...

I never use em to slow down as some of my co-workers do...One of em told me he uses it to slow down on downgrades to save his tractor brakes...I dont buy that one either..I say use em all!!

As far as using trailer brake after a steer tire pop?
Hmmm, have to ponder the physics and facts there, interesting tho

 

This. Especially with spring-loaded pins sometimes this is the only way to get those pins loose... hitting and holding the trailer brake and rocking front and back a little. God I love air-release pins.

 

And extra gaskets for us tanker yankers!