The Tractor Protection Valve: Ensuring Safety on the Road


In combination vehicles, the braking system is equipped with various components that control not only the vehicle’s brakes but also the trailer brakes. These components play a crucial role in maintaining safety on the road.

Trailer Hand Valve

6.2 Combination Vehicle Air Brakes

The trailer hand valve, also known as the “trolley valve” or “Johnson bar,” operates the trailer brakes. However, it should only be used for testing purposes and not while driving to prevent any risk of trailer skidding. Instead, the foot brake should be used, as it activates all the brakes on the vehicle, including the trailer brakes. Using the foot brake significantly reduces the chances of a skid or jackknife incident.

It’s important to note that the hand valve should never be used for parking, as it may cause the release of air and unlock the brakes (applies to trailers without spring brakes). Always engage the parking brakes to secure the trailer in place. If the trailer lacks spring brakes, use wheel chocks as an additional safety measure.

Tractor Protection Valve

The tractor protection valve serves as a vital safety feature in the event of a trailer breakaway or a severe air leak. It is controlled by the “trailer air supply” control valve located in the cab. This valve allows the driver to open or close the tractor protection valve.

The tractor protection valve automatically closes when the air pressure drops to a range of 20 to 45 psi (pounds per square inch). Once closed, it prevents air from escaping the tractor and simultaneously releases the air from the trailer’s emergency line. This action activates the emergency brakes on the trailer, ensuring the vehicle’s stability and safety. (Emergency brakes will be discussed further later in this article.)

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Trailer Air Supply Control

On newer vehicles, the trailer air supply control is a red 8-sided knob that manages the tractor protection valve. Pushing it in supplies air to the trailer, while pulling it out shuts off the air supply and engages the trailer’s emergency brakes. The knob will pop out, closing the tractor protection valve, when the air pressure falls within the 20 to 45 psi range.

Older vehicles may have a lever instead of a knob for controlling the tractor protection valve or “emergency” valves. The lever has a “normal” position used for pulling a trailer and an “emergency” position used to shut off the air supply and activate the trailer’s emergency brakes.

Trailer Air Lines

6.2 Combination Vehicle Air Brakes

Every combination vehicle has two essential air lines: the service line and the emergency line. These lines connect the various components between each vehicle, such as the tractor to the trailer, trailer to dolly, and dolly to a second trailer.

Service Air Line

The service air line, also referred to as the “control line” or “signal line,” carries air that is controlled by either the foot brake or the trailer hand brake. The pressure in the service line varies based on the intensity of the foot brake or hand valve application. The service line is connected to relay valves, which allow the trailer brakes to engage more rapidly than they would otherwise.

Emergency Air Line

The emergency air line, also known as the supply line, serves two purposes. Firstly, it supplies air to the trailer air tanks, and secondly, it controls the emergency brakes on combination vehicles. If there is a loss of air pressure in the emergency line, the trailer’s emergency brakes will activate. This pressure loss can be caused by a trailer breaking loose and damaging the emergency air hose or by the rupture of a hose, metal tubing, or other components, leading to air leakage. When the emergency line experiences a pressure drop, it triggers the closure of the tractor protection valve (indicated by the popping out of the air supply knob).

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To differentiate between the emergency line and the service line, emergency lines are often coded with the color red (red hoses, red couplers, etc.) to avoid confusion with the blue-colored service line.

Remember, understanding the functionality of these crucial components in a combination vehicle’s braking system is essential for ensuring safe and efficient operation on the road.

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