Well Bob, don't feel bad...looks like I got to eat some of my words too....tonight before Rocky gets to call me on them tomorrow-with his "air board" :roll:
pertaining to When/how--those trailer spring brakes dynimite.

I preformed this test: stopped truck on parking lot...released trailer brakes...climbed out and disconnected the SERVICE line..then got back in started rolling about 10 mph and stepped in on cluthc and popped the trailer brakes........she slowly came to a stop.
So, I guess I need to review exactly how that trailer relay works....

Now, I got to say, in 15years of driving these rigs, I've never lost a Supply line or had one come loose while rolling, but I have lost a couple service lines--even broke a supply line hooking up and needed to switch the service over to limp a trailer to the nearest truckstop to save time/miles to get it replaced.
But, I was trained/told there's a trailer protection valve too- that's suppose to prevent the rapid loss of air and prevent the springs from coming on(locking up) if the line gets broken forward of the relay( between it and the tractor)....so I'm looking forward to Rocky's test results tomorrow---

but, I also remember him writing about not know for sure if his tractor protection valve on "his board" was still 100% and not damaged from all that thumb holding down.

I'll try it whenever i get a break and a parking lot on a hillside and see what happens.

 

Here is what "should" happen when you lose air lines connecting the tractor to the trailer.

Both Dash valves are in, supplying air to the tractor springs and the trailer.

If you lose the service line, and make a sustained brake application, the tractor will lose air and eventually the trailer brakes will dynamite, and the tractor will protect (activation of the tractor protection system which will pop out the red dash valve).

If you lose the supply line, the first thing that happens is air loss, which the trailer relay valve senses, then the trailer will dynamite immediately (does not matter if you have spring brakes with a spring brake trailer relay vavle or the old style service brakes with a relay-emergency valve) and then the tractor will protect.

So, in the event, that the trailer disconnects and both air lines are severed, the trailer brakes apply almost immediately, and the tractor protects from air loss so you can bring it to a controlled stop.

Of course, I would think the landing gear hitting the ground will bring the trailer to a pretty quick stop all by themselves.

 

Yeah, but who said anything about the trailer becoming disconnected from the tractor? Unless you got one of those "black dog" key switch disconnects--where you can make a emergency seperation--and try to outrun it or sling it off in a curve.<<---- I just love the Movies--so unreal.

 

I don't feel bad... I haven't eaten any words from this thread, I haven't been given any reason to.

 

I have seen it way too often, and not in the movies.

 

here's another experiment:
(1)park your truck on a hillside- engage the brakes--then supply air to the trailer brakes to dis-engage them(see-not holding in red button as that would damage it)
(2) disconnect the blue service line--- FIRST( SIGNAL AIR )
(3) NOW disconnect the RED supply line-

tractor protection valve should pop when air drops to around 65psi

(4) get in the truck, and release the tractor brakes----get ready to cover the foot brake---your trailer spring brakes shouldn't have engaged( there's a protection valve on the trailer system just like the tractor---
very unsafe to have brakes lock up while rolling due to severed air line.



When you remove the supply glad hand, the trailer dynamites almost immediately. Air bleeds down to about 85 and tractor protects. I was pretty sure it would, but I don't usually do the demo this way. I had to make sure.

 

It is not the tractor protection valve or the tractor protection control valve (Red Dash Valve) that was damaged. It was the dash valve set that I sent the post in from Bendix that was damaged. It was one I had in my tractor and use only for demonstrations in the classroom. I don't have it hooked up to my air board.

 

I am always worried about a glad hand coming off, I squeeze the things to hold them tight but "it" can happen, especially in the very cold temps, air lines get brittle etc.

Driving around on ice 6 months a year, a dynamite could kill someone...

Any ideas ?

 

Guess I just haven't been in the wrong place at the right time

.....I've seen trailers roll--taking the tractor with it--roll the tractor like a gator rolling it's pray-but the tractor stayed connected. seen a tractor nose down in a ditch--trailer sticking straight up in the air--still that kingpin held...even seen drivers drop trailer tandems off in a deep ditch and pull one side of the drives completely off the ground--still that kingpin held.

How much force would that take anyway?

 

99% of the time, they uncouple due to improper 5th wheel adjustment.