When the tractor protection valve closes, it stops any air from going out of the tractor. It also lets the air out of the trailer emergency line. This causes the trailer emergency brakes to come on

Check for enough clearance between rear of tractor frame and landing gear and between the top of the tractor tires and the nose of the trailer.

Go under trailer and investigate the back of the fifth wheel. Make sure the fifth wheel jaws have closed around the shank of the kingpin.

–45 psi, the trailer air supply control knob should pop out, or the valve control should go from the "normal" to the "emergency" position.

The emergency line also called the supply line has two purposes. First, it supplies air to the trailer air tanks. Second, the emergency line controls the emergency brakes on combination vehicles

Be very careful about driving bobtail tractors or tractors without semitrailers. Tests have shown that bobtails can be very hard to stop smoothly and can take longer to stop than a tractor-semitrailer loaded to maximum gross weight.

The height of the vehicle's center of gravity is very important for safe handling. A high center of gravity means you are more likely to tip over.

Make sure air pressure is up to normal.

If you do cross the air-lines, supply air will be sent to the service line instead of going to charge the trailer air tanks. Air will not be available to release the trailer spring brakes. If the spring brakes don't release when you push the trailer air supply control, check the air-line connections.

You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.

When a vehicle goes around a corner the rear wheels follow a different path than the front wheels, and this is called off-tracking or "cheating." Longer vehicles will off-track more. If there is more than one trailer, the rear wheels of the last trailer will off-track the most.

It may be difficult to tell if some units are equipped with ABS. Look under the vehicle for the electronic control unit (ECU) and wheel speed sensor wires coming from the back of the brakes.

More than half of truck driver deaths in crashes are the result of truck rollovers.

Make sure air and electrical lines are not tangled, pinched, dragging against tractor parts, and have enough slack for turns.

The service line, also called the control line or signal line, carries air controlled by the foot brake or the trailer hand brake.

’s specification usually 20 – 45 psi. This will cause the spring brakes to come on.

With a fully-charged air system, turn off the engine, release the parking brake; and time the air pressure drop. The loss rate should be less than three psi in one minute for combination vehicles, adding one second if the brakes are applied.

The emergency line, also called the supply line, has two purposes. First, it supplies air to the trailer air tanks. Second, the emergency line controls the emergency brakes on combination vehicles.

If you are driving a truck or bus that cannot make the right turn without swinging into another lane, turn wide as you complete the turn. Keep the rear of your vehicle close to the curb. This will stop other drivers from passing you on the right.

’s specification usually 20 – 45 psi.

The heavier the vehicle, the more work the brakes must do to stop it, and the more heat they absorb. But the brakes, tires, springs, and shock absorbers on heavy vehicles are designed to work best when the vehicle is fully loaded. Empty trucks require greater stopping distances because an empty vehicle has less traction.

According to federal regulations, if the load projects more than 4 feet behind the rear of the vehicle, the end of the load must be marked as follows: (1) with red flags, each 18 inches square, in the daytime; or (2) with two red lights at night.

The tractor protection valve keeps air in the tractor or truck brake system should the trailer break away or develop a bad leak. The tractor protection valve is controlled by the trailer air supply control valve in the cab. The control valve allows you to open and shut the tractor protection valve. The tractor protection valve will close automatically if air pressure is low (in the range of 20 to 45 psi). When the tractor protection valve closes, it stops any air from going out of the tractor. It also lets the air out of the trailer emergency line. This causes the trailer emergency brakes to come on, with possible loss of control.

Check glad hand seals and connect tractor emergency and service air-line to trailer emergency and service glad hand. Make sure air-lines are safely supported where they won't be crushed or caught while tractor is backing under the trailer.

Make sure air pressure is up to normal before backing under the trailer.

The trailer hand valve, also called the trolley valve or Johnson bar, works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes.

Make sure there is no space between upper and lower fifth wheel. If there is space, something is wrong, the kingpin may be on top of the closed fifth wheel jaws, and trailer would come loose very easily.

Trailers will have yellow ABS malfunction lamps on the left side, either on the front or rear corner.

Check if fifth wheel is in proper position for coupling: The fifth wheel should be tilted down toward rear of tractor.

Trucks with trailers have a dangerous crack-the-whip effect caused by rearward amplification. When you make a quick lane change, the crack-the-whip effect can turn the trailer over. There are many accidents where only the trailer has overturned.

If there are no dummy couplers, the glad hands can sometimes be locked together. It is very important to keep the air supply clean and free of moisture.

Converter dollies and trailers built before 1975 are not required to have spring brakes.

If trailer is loaded, after the landing gear makes firm contact with the ground, turn crank in low gear a few extra turns. This will lift some weight off the tractor making it easier to unlatch fifth wheel, and to recouple to the trailer.

If there is more than one trailer, the rear wheels of the last trailer will off-track the most

Large combination vehicles take longer to stop when they are empty than when they are fully loaded. When lightly loaded, the very stiff suspension springs and strong brakes give poor traction and make it very easy to lock up the wheels.

Shut off trailer air supply to lock trailer brakes. Ease pressure on fifth wheel locking jaws by backing up gently, applying the parking brake while tractor is pushing against the kingpin.

Wait until the air pressure is normal, then shut engine off so you can hear the brakes. Check brake system for crossed air-lines by: applying and releasing trailer brakes and listen for sound of trailer brakes being applied and released. You should hear the brakes move when applied and air escape when the brakes are released.

One good rule says you need at least one second for each 10 feet of vehicle length at speeds below 40 mph. At greater speeds, you must add 1 second for safety.

The trailer hand valve, also called the trolley valve or Johnson bar, works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid.

Pull out the air supply knob or move the tractor protection valve control from normal to emergency.