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  #21  
Old 02-08-2007, 10:13 PM
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quote="JoeyB"]
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No, they must BOTH pop at 45-20, although not necessarily at the same time.
State your source

Birken
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
Read what I have written again- that's what I'm saying, bob. The trailer goes into emergency on it's own without any action from the tractor OTHER THAN low pressure in the supply line.
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  #22  
Old 02-08-2007, 10:51 PM
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All right, Mr. Lawyer... I read that thing again and I see what you're saying. That's talking about the red knob, we're talking about the yellow knob. I used to have a copy of the ATA recommended practice, I'll be able to get my hands on it saturday. We'll see if that sheds any light, I'm sure they detailed an air brake test.

All of our texts at the school says both valves pop 45-20, and that's how we taught it, and that's usually how it worked.
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  #23  
Old 02-09-2007, 04:02 AM
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Most any training material will say 20-65 psi or thereabouts for yellow knob pop-out but they do that to cover all the years, makes and models from the brand new to the ancient. However I am just saying from experience and from the Bendix air brake class put on by Paul Jones, that on newer trucks sometimes the valve will not pop out until the system is almost at zero pressure. This is a good thing because the driver can still pull that knob whenever he wants to and it is better for him to retain manual control over the brakes of the truck than to have it come on all at once (a "dynamiter") if it can be avoided.

Also if system pressure drops below about 20 psi or so the spring brakes will be on pretty much full anyway so it doesn't really matter whether the yellow knob is out or in at that point.

Finally does anyone remember the old Ford C-series trucks with the modified TW (flipper style) valve for the parking brake? These were made into the 1990s and had no provision whatsoever for automatically changing the position of the valve based on air pressure; the valve could be moved into either position even with no air whatsoever in any tank. I actually liked those

Birken
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  #24  
Old 02-09-2007, 12:49 PM
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this applies to mv-2, mv-3, and midland kn20615 tractor dash valve systems ;

quote* "Loss of pressure in both systems- if pressure in the highest system is lowered between approximately 35 and 45 psi, the trailer air supply valve (red button) will "pop", causing the trailer spring brakes to apply. If pressure continues to drop so that pressure in the highest system is between approximately 20 and 30 psi the park brake control (yellow button) will "pop", causing the tractor spring brakes to apply." *unquote

i don't necessarily agree with this entirely, but what kind of valve do you have?
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  #25  
Old 02-09-2007, 01:29 PM
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as far as myself.. i have the red/ yellow dash knobs.
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  #26  
Old 02-10-2007, 05:35 AM
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Well one I know for a fact of the top of my head doesn't really pop until basically zero PSI is a Bendix PP-DC which is like half a MV-3. If I recall correctly the regular PP valves (PP-1?) are available with all sorts of different springs including no spring at all.

I have not seen a Midland or Meritor valve that did not have a pretty hefty spring in it though.

Birken
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  #27  
Old 02-12-2007, 12:07 PM
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Quote:
Originally Posted by Birken Vogt
Well one I know for a fact of the top of my head doesn't really pop until basically zero PSI is a Bendix PP-DC which is like half a MV-3. If I recall correctly the regular PP valves (PP-1?) are available with all sorts of different springs including no spring at all.

I have not seen a Midland or Meritor valve that did not have a pretty hefty spring in it though.

Birken
pp-dc is a tractor park valve, and is designed to pop off at 20-30 psi

i believe that the pp- 1, 2, 3, 5, 7, and 8 all have different pop off settings
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  #28  
Old 02-13-2007, 02:31 AM
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The pp-1, 2, 3, etc. are all different designs of valves for different applications, models if you will with different ports and functions. They could all have the same pop pressure if ordered that way, the number is a model number and not anything to do with pressures, etc. The pp-1 is the most common for tractor parking (or at least the "classic") and is available with many different springs for different pop pressures, or no spring at all.

Birken
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  #29  
Old 02-13-2007, 03:37 AM
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The purpose for a dash valve to pop out (dynamite) is as follows.

If you lose your air pressure, then at some point your spring brakes will start to drag. That is usually around 60 to 70 psi (and that is of course why your governor MUST start to reload at a minimum of 80 psi).

If you get down to 10 psi and the yellow dash valve has NOT popped out, then in the event that you forget to pull it and start the truck up, and walk away from it, then it could very possibly roll away once it has built up enough pressure to release the brakes.

That is why they got rid of manual dash valves.

In BC, which has the same rules as all other jurisdictions in North America, if your Yellow (tractor) or Red (trailer) dash valves do not pop out by 20 psi, it is an out of service defect, and you will be parked.
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  #30  
Old 02-13-2007, 06:54 AM
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o.k rky mountn.. now.. that makes sense.. t.y.
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