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.
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