What do you use the trolley valve for?

Testing service brakes on trailers, sliding tandems, stretching extendable trailers, sliding fifth wheel.

 

It's really nice if you have a manual and are stopped on a hill. You hold the trailer while you get the rig to grab a gear and get going. Used it frequently in SF. Twenty years ago it was part of the driving test in CA.
I haven't given this a try yet but an instructor was telling me that in winter he uses the trailer break for light braking instead of the pedal. His reasoning is that since the rear is braking there is less of a chance of jack knife not on ice.

 

It does stretch the truck but personally I would try a different method. Power brake off slick hills. More axles braking while providing traction simultaneously.

 

You can test the service brakes on a trailer by applying the parking spring brakes? (No you can’t). Some of our stretch trailers you have to use the hand valve to stretch. I’m the one who redesigned the air system on those trailers. You absolutely can not use the red brake valve to replace the a trailer hand valve.

 

Straighten things out when the trailer starts pushing.

 

Hahaha!!! No, you can’t. Also it is a hell of a lot easier on the clutch when sliding anything when you get rolling a little then slide it.

 

My newish Utility Dry Van has the air operated locking pins on the slider, I had a tool that slides behind the button to lock it out with the brakes released so I can move it while moving by using the johnson bar. Saving a ton of miles on my clutch.

 

Coming down steep hills in the bush, I use it multiple times per day. I'll use it while on the foot brake even, to get the trailer brakes applying harder than the trucks. Mud, snow, ice, and the road is only 4' - 8' wider than the truck is, if you lock a truck tire for a half second you can go off the road, and you will roll over many times before you stop rolling. You can maybe, and i emphasize maybe get away with a split second of trailer lock up. When you're heavy, on steep terrain, and traction is minimal, you'll love that handle.

Another scenario is at speed, and the corner isn't going well, you can grab it hard enough to lock up your trailer for a half second, making the trailer slide, and help pivot around the corner.
Don't do this on busy roads, I mean on radio controlled logging roads where I know what is coming at me, and if nothing is, I own the whole road from snow bank to snow bank, and I use it all. I would refuse to drive a truck offroad without the magic lever.

Anyone who thinks that popping the parking brake button to apply the spring powered parking brake is the same, actually makes me nervous that they drive a truck.