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The trailer supply valve is an on/off switch that supplies air to the trailer.The system consists of the tractor protection valve and the trailer supply valve.

Updated Jan 2021

Prevents total loss of air from the tractor if the trailer breaks away from the tractor or if the connection lines between the tractor and the trailer becomes separated or ruptures.

When the engineers named this part of the air brake system, it was still early in the morning; in other words, they hadn't been drinking yet.

The Tractor Protection System "protects the tractor's air supply" in the event of a catastrophic air loss in the semi trailer or any other trailer fitted with air brakes.

In most cases, the semi-trailer falls off the back of the truck causing a catastrophic air loss through the gladhands.

Tractor Protection System

The Tractor Protection system consists of two valves:

1) Trailer Air Supply Valve.

2) Tractor Protection Valve

Unfortunately, the engineers had been imbibing when they named the tractor protection valve.

"We'll screw every one up!

"Let's call the system and the valve the same name!"

So...there's the Tractor Protection System and the Tractor Protection Valve.

And the tractor protection valve is part of the tractor protection system.

The Trailer Air Supply Valve is also the part of the Tractor Protection System.

Tractor Protection Valve

Think of the system as follows:

The Tractor Protection Valve is the guard or sentry at the entrance to the military base.

It monitors air flow in and out of the trailer; in a similar manner that a guard monitors traffic in and out of the military base.

If the tractor protection valve detects catastrophic air loss in the trailer--like the guard at the military base when the terrorists show up--it hits the panic button.

The panic button in this case is the trailer air supply valve.

The air lines at the back of the towing unit are plumbed into the tractor protection valve which acts as a guard and monitors air pressure into the trailer.

The tractor protection valve, in an emergency, tells the trailer air supply to shut off air to the trailer when system pressure is drops between 20psi (138kPa) and 45psi (310kPa).

Testing the Tractor Protection Valve

Although not a required test in Ontario, the trailer air supply valve must shut off air to the trailer between 20psi (138kPa) and 45psi (310kPa).

With the truck (parking brake) and trailer (air supply) spring parking brake buttons pushed in on the dash, pump the brake pedal to lower the system air pressure.

When the system air pressure is between 20psi (138kPa) and 45psi (310kPa), the trailer air supply valve should pop automatically, shutting off air to the trailer.

Trailer Air Supply Valve

The trailer air supply valve is comparable to a light switch.

The light switch does not directly turn off and on the light, but rather controls the flow of electricity to the light.

As a light switch turns off and on electricity, the trailer air supply valve turns air on and off to the trailer.

Like the light switch, the trailer air supply controls the air supply to the trailer, thus trailer air supply valve.

By turning on and shutting off the air supply, the trailer air supply valve should also control the trailer's spring brakes.

Like a light switch in the living room that controls the light in the garage, a person actually has to go the garage to ensure that the light switch did indeed turn off the light.

Like the light in the garage, the driver--student in this case--has to go to the trailer to determine that the spring parking brakes have in fact applied.

How We Know the Trailer Brakes Are Applied

In most cases, the push rod and slack adjuster will form an approximate 90° angle when the spring parking brakes are applied on the trailer.


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