As far as I know the yellow valve does not really have to pop until basically 0 psi and many trucks are set up this way. The reason being you want to have manual control of your tractor brakes as long as possible in an emergency situation and not have a "dynamiter" where the spring brakes all set up simultaneously. Which could definitely cause loss of control if you are still trying to get the thing under control while moving at a pretty good clip. This is not loss of a safety feature as the spring brakes will still come on gradually as the pressure drops below 40 psi or so anyway but you want them to stay off as long as possible to give you every chance you've got to fix whatever is wrong yourself.

Now the red valve on the other hand, is supposed to pop somewhere between 45 and 60 IIRC, too lazy to look it up right now, but always well before the yellow knob does to close the tractor protection valve and stop catastrophic air loss through the trailer.

And of course many yellow knobs will pop at pressures substantially higher than zero, there are 10psi, 20 psi, 40 psi, and even higher available, it all depends what you buy and install.

Birken

 

No, they must BOTH pop at 45-20, although not necessarily at the same time. As far as their poping with the engine running, that gets back to what I said earlier. Just a hair too much internal drag in the valve and no dice. But add the slightest vibration to the equation and presto!

Don't know for sure, but I'd guess that as long as you can get them both to pop, even if it's near zero, you're good to go with the DOT. The only reason I say that is because if it were any other way, half the trucks in the country would be O/S.

 

i agree with birken here; the tractor protection valve (breakaway valve) shuts off air supply to the trailer in the event of trailer air system failure, and stops it from depleting the tractor air tanks

red button should pop well before the yellow

 

State your source

Birken

 

bob & all the rest.. i really appreciate all your time & input here. i over a short time i put in a new dash valve. then i got a used one. then last week another new one. all have performed the same. somehwere around 5 lbs before poping out. they both pop out at same time every time. i guess this is one of those things that keeps ya scratching your head..

 

Federal Motor Carrier Safety Regulations 393.43 (b) ..."One of these means shall operate automatically in the event of reduction of the towing vehicle air supply to a fixed pressure which shall not be lower than 20 pounds per square inch nor higher than 45 pounds per square inch"...

Trailers will usually pop before the red knob does; that's the trailer's relay emergency valve response to the lowered supply pressure from the red gladhand. The trailer does this on it's own. If you keep fanning off air, usually the trailer supply knob will pop next, and then the yellow one.

 

thank you joey...

 

Not trying to be a lawyer here, but 393.43(b) applies to trailer brakes only. There is no reference to the tractor emergency/park system at all.

Right or wrong, the trend for the yellow knob on newer trucks has been for it to pop just above zero PSI, or not at all due to o-ring drag, in order to give the driver the most chance possible to control the vehicle manually. In the event of catastrophic loss of air the spring brakes will come on anyway so what's the difference.

 

thank you birken

 

quote="JoeyB"]Federal Motor Carrier Safety Regulations 393.43 (b) ..."One of these means shall operate automatically in the event of reduction of the towing vehicle air supply to a fixed pressure which shall not be lower than 20 pounds per square inch nor higher than 45 pounds per square inch"...

Trailers will usually pop before the red knob does; that's the trailer's relay emergency valve response to the lowered supply pressure from the red gladhand.

there is no trailer valve that causes the red button to pop off. the valve pops off based on line pressure ; when pressure over the piston of the red pp valve is reduced to 20psi (20 to 45) the spring force under the piston pushes it out. that dumps emergency line pressure and then closes the tractor protection valve to isolate the trailer air system from the tractor

The trailer does this on it's own.

the tractor controls the air supply and service function to the trailer, the trailer only reacts to what it is told by the tractor

If you keep fanning off air, usually the trailer supply knob will pop next, and then the yellow one.[/quote

agreed