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.

 

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.

 

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

 

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.

 

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.