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.

 

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.

 

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

 

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?

 

as far as myself.. i have the red/ yellow dash knobs.

 

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

 

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

 

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.

 

o.k rky mountn.. now.. that makes sense.. t.y.