park brake

[JoeyB]

quote="JoeyB"]

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.

[JoeyB]

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.

[Birken Vogt]

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

[bob h]

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?

[heavyhaulerss]

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

[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

[bob h]

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

[Birken Vogt]

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

[RockyMtnProDriver]

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.

[heavyhaulerss]

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

[JoeyB]

Well I got hold of a copy of a Recomended Practices Manual (The Maintenance and Technology Council of the ATA), but it's from 2000-2001. RP 619A titled "Air System Inspection Procedure" goes into great detail with regard to testing the air system on the entire rig. They do look for the 45-20 on the tractor protection valve (the red one), but on that one there's no argument. As for the Tractor Parking valve, they just say to verify that the spring brakes apply when you pull it out and release when you push it in. There's not a word about any specific pressure.

Personally, I like Rocky's explanation. That makes perfect sense and dovetails with the idea of it's staying in as long as possible to keep control with the driver.

[RockyMtnProDriver]

One of my buddies is a Commercial Vehicle Inspector for the BC government and the next time I talk to him I am going to get the straight goods from him as to what the law says.

[bob h]

inspection criteria here says that the trailer valve must pop by 20 psi, they seem to be referring to the tractor protection valve, but they do specify that the button should pop before 20 psi... there was no reference to the tractor park valve at all

... i tried an fld today, the red popped at 45 psi, the yellow at just above zero

[RockyMtnProDriver]

For those that want it here is how a tractor protection system works.

The tractor protection control valve (red dash valve) when pushed in (on modern trucks, some older systems had them pull out to supply air) sends air to the supply side (usually red air line out of the back of the tractor) through the tractor protection valve.

Air is also directed inside the tractor protection valve to the bottom of a sliding valve. As this valve is pushed up it allows air to transfer from the service side (foot valve) to the service line going to the trailer (usually a blue line) when a brake application is made with either the foot valve or hand valve.

When the supply line is disconnected, or ruptured, and enough air is lost, then there is not enough air to hold up the sliding valve and it is forced down by a spring.

Once this happens, the tractor protection control valve (red dash valve) then pops out as there is not enough air to hold it in.

And because it has slid down, it seals up the service side and no air can travel from the foot valve to the trailer on the service side.

When you break a service line (control line) nothing will happen until you make a brake application.

Then you have air loss, and when enough air is lost the trailer brakes will dynamite. If you have spring brakes on the trailer, the spring brakes come on as air is evacuated from the spring chamber. If you have service brakes, air is blown into the service chamber to make the brakes come on immediately. This is where the term dynamite comes from. once it gets to the appropriate air pressure (45 to 20) the tractor protection system activates and the air is shut off at the red dash valve.

There is a tractor protection system where the brakes will dynamite immediately upon the rupture of the service line. This is more common in older logging trucks, although some are still built this way.

When you break a supply line, the trailer dynamites the moment the trailer relay valve (spring or service) senses air loss, and once you get to the appropriate air pressure (45 to 20) the tractor protection system activates and air is shut off at the red dash valve.

Of course, if your trailer dynamites while you are driving at speed, then you either will skid your tires in the summer, or in the winter the trailer will try and pass your tractor.

Now, more than likely if you break both of the lines, it is because your trailer is no longer attached to your tractor.

And you have larger problems than broken air lines.

[RockyMtnProDriver]

I talked today to my buddy from the CVI. He checked and it appears that there is NO regulation that a tractor spring brake dash valve has to pop out at any PSI whatsoever.

They use to, but for some reason, they do not need to anymore. I think this is where I confused old regs and new regulations.

He was unsure if there is specification that manufacturers have to meet when they are brand new.

He was quite surprised to find that out.

He said that Trailer valves MUST pop out no lower than 20, and can range as high as 45 in the regulations, but many come out much higher than that.

All jurisdictions on North America comply to the same regulation, so it must be the same in Canada, USA, and Mexico.

He said he was going to dig a bit deeper as it surprises him especially taking into accout that someone could walk away from a tractor with the dash valve in, not enough air in the tanks to release the springs and if the engine was running when it built up enough air it could roll away.

It does not seem like a very safe thing to him. Or me, for that matter.