The trailer air supply control on newer vehicles is a red eight-sided knob, which you use to control the tractor protection valve. You push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes.

From cab, push in air supply knob or move tractor protection valve control from the emergency to the normal position to supply air to the trailer brake system.

To test the trailer spring brakes, you need to have the service brake off or released, parking brake off, trailer air supply out shutting the door to the trailer, and trolley released.

’s specification usually 20 – 45 psi.

Glad hands are coupling devices used to connect the service and emergency air-lines from the truck or tractor to the trailer.

Hang electrical cable with plug down to prevent moisture from entering it.

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

Brake the same way, regardless of whether you have ABS on the tractor, the trailer, or both.

The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it. Raise or lower the trailer as needed. If the trailer is too low, the tractor may strike and damage the trailer nose; if the trailer is too high, it may not couple correctly.

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.

When more cargo is piled up in a truck, the "center of gravity" moves higher up from the road. The truck becomes easier to turn over. Fully loaded rigs are ten times more likely to roll over in a crash than empty rigs.

The earliest way to notice that your trailer is jackknifing is to see it swinging out in your mirrors.

Rear wheel braking skids occur when the rear drive wheels lock. Because locked wheels have less traction than rolling wheels, the rear wheels usually slide sideways to "catch up" with the front wheels. With vehicles towing trailers, a drive-wheel skid can let the trailer push the towing vehicle sideways, causing a sudden jackknife.

To unlock the fifth-wheel pull the release handle to "open" position.

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.

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.

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.

To test the trailer air brake system, first charge it with air. Push in the red trailer air supply knob or place the tractor protection valve control in the "normal" position. Wait until the air pressure reaches normal before you start your tests.

The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it. If the trailer is too low, the tractor may strike and damage the trailer nose; if the trailer is too high, it may not couple correctly.

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.

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

Check that the locking lever is in the lock position. If the locking lever does not go into the locked position the coupling is not complete.

Pull tractor forward until fifth wheel comes out from under the trailer but stop with tractor frame under trailer preventing the trailer from falling to ground if landing gear should collapse or sink.

If the trailer has no spring brakes you would be able to drive away, but service breaks for the trailer only will not work.

When the wheels of a trailer lock up, the trailer will tend to swing around. This is more likely to happen when the trailer is empty or lightly loaded. This type of jackknife is often called a "trailer jackknife." Applying the trailer brakes would extend the skid and trailer jackknife.

As part of your Vehicle inspection, make sure the truck is not overloaded and the cargo is balanced and secured properly.

A major leak in the emergency line will cause the tractor protection valve to close and the trailer emergency brakes to come on. But the brakes will hold only if there is air pressure in the trailer air tank.

Trailers that do not have spring brakes have emergency brakes, which work from the air stored in the trailer air tank. A major leak in the emergency line will cause the tractor protection valve to close and the trailer emergency brakes to come on, but the brakes will hold only if there is air pressure in the trailer air tank. Eventually, the air will leak away and then there will be no brakes. Therefore, it is very important for safety that you use wheel chocks when you park trailers without spring brakes.

Before inspection the trailer while coupling you need to secure the vehicle. Put transmission in neutral. Put parking brakes on. Shut off engine and take key with you so someone else won't move truck while you are under it.

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.

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 for normal air pressure, release the parking brakes, release the service brake, make sure the trailer air supply is in / open, move the vehicle forward slowly, and apply trailer brakes with the hand control or trolley valve. You should feel the brakes come on. This tells you the trailer brakes are connected and working.

The following two things will help you prevent rollover--keep the cargo as close to the ground as possible and drive slowly around turns.

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. Never use the hand valve for parking because all the air might leak out unlocking the brakes.

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, we do not add the additional second here because the brakes are released.

Use low gear range to begin raising the landing gear, once free of weight, switch to the high gear range. Raise the landing gear all the way up. Never drive with landing gear only part way up as it may catch on railroad tracks or other things. After raising landing gear, secure the crank handle safely.

Raise trailer landing gear slightly off ground. Pull tractor gently forward while the trailer brakes are still locked to check that the trailer is locked onto the tractor.

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.

Be sure that the fifth wheel plate is greased as required. Failure to keep the fifth wheel plate lubricated could cause steering problems because of friction between the tractor and trailer.

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.

Use lowest reverse gear. Back tractor slowly under trailer to avoid hitting the kingpin too hard, and stop when the kingpin is locked into the fifth wheel.

Shut the engine off so you can hear the brakes. Apply and release trailer brakes and listen for sounds of trailer brakes being applied and released. You should hear the brakes move when applied and air escape when the brakes are released.

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.

Make sure surface of parking area can support weight of trailer.

From cab, push in air supply knob or move tractor protection valve control from the emergency to the normal position to supply air to the trailer brake system. Check brake system for crossed air-lines. Shut engine off so you can hear the brakes. Check air brake system pressure gauge for signs of major air loss.

Some vehicles have "dead end" or dummy couplers to which the hoses may be attached when they are not in use. This will prevent water and dirt from getting into the coupler and the air-lines.

If for any reason you get stuck on the tracks, get out of the vehicle and away from the tracks. Check signposts or signal housing at the crossing for emergency notification information. Call 911 or another emergency number. Give the location of the crossing using all identifiable landmarks, especially the DOT number, if posted.

The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it. Raise or lower the trailer as needed. If the trailer is too low, the tractor may strike and damage the trailer nose; if the trailer is too high, it may not couple correctly.

Once you realize your vehicle is in a skid, you should release the brakes so that the wheels can grip the road again.