Because the trailer still has service brakes with or without the hand valve.

 

Yes, I understand that, but why would you want to limit your options, when just by changing the plumbing you increase them? Thats my point.

 

Have you thought about this possibility:

poor dumb limited experience driver's service line comes loose from glad hand or gets cut(what ever)
he's rolling down steep hill, loosing air( but can't figure out WHY) he's pulling down hard on that hand valve----got his foot on the brake pedal.
Do you really want him to bleed off all his air from both tanks?

 

If he is going slow enough, then the springs will apply when the air is lost and the truck will stop.

If he is going to fast, then brake fade will occur and he wont have any brakes anyway.

But, that does not take away from the fact that you can have it plumbed into Blended, Primary or secondary.

It is almost as if they are saying, it does not matter.

I have racked my brain for it, but cannot come with an answer that applies.

Except, that a hand valve is not a necessary part of an air brake system, so as an add on, no thought was put into how, or probably more realistically, no thought was put into why.

There is a guy I am going to ask, he might know.

 

but the spring brakes on the trailer won't apply until the air pressure in the trailer tank is depleted....

 

If the tanks are being depleted at the same time, the springs will start to drag about 70 psi and by 40 will be on mostly.

At 40 to 20 your dash valves will dynamite and on they come.

BUT.....if this is happening at speed, then they are going to get hot, brake fade will start to effect the linings ability to contact the drums and you wont have brakes anyway. This assuming you are still going down that steep grade you where talking about.

Flat ground, if the ground is dry, you will probably just eventually stop. If it is wet, your tractor could jackknife if the tractor springs come on harder than the trailer, or the trailer could swing out if the trailer springs come on harder than the tractors.

 

It is impossible to deplete the tractor air supply with an air leak from behind the tractor... that is what the tractor protection valve is for.

 

Bob, You,re wrong:

preform the following experiment:

with the tractor running, set the tractor brakes, supply air to your trailer as
if you were ROLLING----now, take your seat belt and wrap around the hand valve like you would if you were checking your trailer brake lights---now go disconnect the blue- service line---and watch your air tanks
deplete----
now, in my case--only the secondary tank empties out----

the trailer brakes will not dynamite unless the air is emptied from the trailer supply tank---which won't happen because there's no signal air to the control valve back there.

 

I did the following test on my air board.

With the hand valve getting is supply from Blended Air, and with the service line disconnected to the trailer, by making a brake application with the hand valve, both Primary air and Secondary air tanks depleted, Trailer tank did not deplete, and trailer dynamited when it got down to about 50 PSI.

With the hand valve getiing its supply from the Secondary tank, secondary tank depleted and trailer did NOT dynamite.

With the hand valve getting its supply from the Primary tank, Primary tank depleted and trailer did NOT dynamite.

 

Well bob's only part wrong,
the tractor protection valve will only engage when there's total air loss---from both tanks--when they both near 65pounds or so--that's when the protection valve on my truck kicks in--
I don't believe I have a way to drain just my primary so it could be triggered by it alone.(maybe)
Trailer brakes dynamiting and protection valve engaging are 2 different things.

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.

this is the reason WHY I can't understand why anyone would want an Aux trailer braking system to feed off both your primary & secondary tanks---
that secondary is a (reserve)---if you were to loose brakes on the trailer---by way of broken service line, etc You'd want to keep as much air in the primary as possible---sure, every time you step on the foot brake you'd be losing air from both----but why would you want to compound the problem by piping air from Both tanks to a trailer brake via a hand valve? what ADVANTAGE would it really serve.