In mix vehicles, the braking system has actually parts to control the trailer brakes, in addition to the parts explained in ar 5. These components are explained below.

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Trailer Hand Valve

The trailer hand valve (also referred to as the “trolley valve” or “Johnson bar”) functions the trailer brakes. The trailer hand valve must be offered only to check the trailer brakes. Do not usage it in driving due to the fact that of the danger of make the trailer skid. The foot brake sends out air come all the brakes ~ above the automobile (including the trailer(s). Over there is much less danger of bring about a skid or jackknife once using just the foot brake.

Never usage the hand valve for parking because all the air could leak out, unlocking the brakes (in trailers that do not have actually spring brakes.) always use the parking brakes when parking. If the trailer does not have actually spring brakes, use wheel chocks to store the trailer native moving.

Tractor defense Valve

The tractor defense valve keeps waiting in the tractor or truck must the trailer break far or develop a bad leak. The tractor defense valve is controlled by the “trailer waiting supply” regulate valve in the cab. The regulate valve allows you come open and shut the tractor defense valve. The tractor protection valve will certainly close automatically if air pressure is low (in the range of 20 to 45 psi). As soon as the tractor defense valve closes, the stops any kind of air from going the end of the tractor. It additionally lets the air out of the trailer emergency line. This causes the trailer emergency brakes come come on. (Emergency brakes are covered later.)

Trailer Air supply Control

The trailer air supply regulate on more recent vehicles is a red 8-sided knob used to manage the tractor defense valve. Press it in to supply the trailer through air, and also pull it the end to shut the waiting off and also put ~ above the trailer emergency brakes. The valve will pop out, thus closing the tractor security valve, once the air press drops right into the variety 20 to 45 psi. Tractor defense valve controls or “emergency” valves on larger vehicles may not operate automatically. There may be a bar rather than a knob. The “normal” place is supplied for pulling a trailer. The “emergency” place is offered to close up door the waiting off and also put top top the trailer emergency brakes.

Trailer wait Lines (see number 6-6)

Every combination vehicle has actually two wait lines: the service line and the emergency line. Castle run in between each vehicle (tractor to trailer, trailer to dolly, dolly to 2nd trailer, etc.).

Service air line:

The business line (also referred to as the “control line” or “signal line”) dead air regulated by the foot brake or the trailer hand brake. Depending upon how hard you push the foot brake or hand valve, the pressure in the business line will similarly change. The service line is connected to relay valves. These valves permit the trailer brakes to it is in applied much more quickly than would certainly otherwise it is in possible.

Emergency waiting line:

The emergency line (also dubbed the supply line) has actually two purposes: (1) To supply air to the trailer waiting tanks, and (2) to manage the emergency brakes on mix vehicles. Loss of air pressure in the emergency line causes the trailer emergency brakes to come on. The push loss can be resulted in by a trailer breaking loose, therefore tearing personal the emergency wait hose. Or it might be led to by a hose, steel tubing or other part that breaks, letting the waiting out. As soon as the emergency heat loses pressure, it also causes the tractor defense valve come close (the air supply knob will pop out).

Emergency lines are frequently coded v the shade red (red hose, red couplers or various other parts) to store from obtaining them mixed up v the blue company line.

A question may come up around the ideal use of the trailer hand valve (trolley valve). Remember, the trailer hand valve must never be provided when driving and also should only be provided to test the trailer service brakes.

Remember: The tractor defense valve, i m sorry controls the trailer emergency brakes will certainly will pop out once air press drops between 20 and 45 psi.

The tractor security valve is designed to activate the trailer emergency brakes before activating the tractor emergency brakes as soon as air press drops also low.

the is very important you understand what the service line does in an waiting brake system. The service line controls the "regular" brakes once you usage the brake pedal. Make sure you fully understand the explanation below.
just as essential as the service line you should fully understand the below explanation of just how the emergency line functions on an air brake system.
once the tractor defense valve closes, the red "emergency brake" valve in the cab of the truck will certainly pop out. This will certainly stop any air native flowing the end of the tractor and also into the trailer. That way, if there is a significant air leak resulting in air push loss in the system, the trailer brakes will certainly activate before the tractor brakes, help you keep some manage of the vehicle.
Any combination of vehicles v a gross linked weight rating (GCWR) of 26,001 or an ext pounds, giving the gross vehicle weight rating (GVWR) of the automobile being towed is in overfill of 10,000 pounds.Any single vehicle through a GVWR the 26,001 or more pounds, or any kind of such car towing one more not in overabundance of 10,000 pounds.Any vehicle, nevertheless of size, design to deliver 16 or an ext persons, consisting of the driver.Any auto required by commonwealth regulations to be placarded while carrying hazardous materials.

Combination Vehicle:

A vehicle with 2 separate parts - the strength unit (tractor) and the trailer. Tractor-trailers are considered mix vehicles.


Hours that Service

HOS refers to the logbook hrs of service regulations.

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Operating while Intoxicated


When a violation by either a driver or company is confirmed, an out-of-service order gets rid of either the driver or the car from the roadway until the violation is corrected.