[bob h]

Originally Posted by RockyMtnProDriver

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.

What valve does your board use for trailer spring brake control?

[RockyMtnProDriver]

Originally Posted by bob h

What valve does your board use for trailer spring brake control?

PP7, but I have hooked up every type, and it makes no difference when you simulate. The end result is always the same.

I talked to my mechanic buddy this morning about where supply air is taken for the hand valve.

His reason for it not being mandated is because it is an auxiliary braking system so it could be Blended, Secondary or Primary.

His thinking was that it would not be best from Blended because if you lost the line between the hand valve and the two way check valve, then you would deplete both tanks.

It would not be best from the primary either, because if you lost the line between the hand valve and the Primary tank, you would deplete the Primary tank and then lose your service brakes on the drivers which would probably be 4 type 30's and would supply a lot of stopping power.

It would have the least effect if you lost it if it was hooked to the Secondary as then all you would lose is the front axle service brakes, which only supply about 5% of your braking power.

So, if your low air comes on, and your primary air is depleting, it just might be from the hand valve. Which is a really good reason to get off the road and find out where the air leak is.

Also, it would be a good idea to check your system so you would know where that air is coming from.

I am going to incorporate a module in my air brake course and show them what would happen in all three cases.

[headborg]

Ok, did some more research--seems I'm half right, half wrong.

The key difference is between: dynimiting the spring brakes---which can only be done--by pulling the knob on the dash with both lines connected to the trailer---this signals an immediate uncontrolled(unregulated) purge of the trailer reservoir(s)----and the spring brakes lock up---if you're rolling when this happens(you're a dumb ass--or really desperate to stop- and could loose control)

otherwise; with (-) pressure detected(i.e. a supply line coming off-air trying to go forward from the emergency valve) and/or no signal(control) pressure detected-the trailer emergency valve activates---which causes a 1-2(the 2 seems a bit long but that's what I read) second CONTROLLED purge--causing the emergency spring brakes to engage.

there seems to be:

emergency relay valve
multi-relay valve
supply dump valve
spring brake control valve
multiple single circuit protection valves
automatic drain valve
safety valve
check valve
unloader valve
differential protection valve

In any event I was wrong about the trailer rolling( very far )---and this seems to indicate that if:
a service line were loose/disconnected or a loose of air- a driver could immediately pull the Red protection control valve and then there would be a controlled application of spring brakes on the trailer--apply them safely and get to the shoulder----
But, if the loss of air were somewhere else and his service line was still connected and he pulled the KNOB(clearly marked in shape of STOP) he might be really dynimiting his brakes---how would he know?

reference;
Heavy Vehicle Technology by David John Lemming, Reg Hartley