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.

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 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.

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.

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.

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.

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.

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 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.

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.

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.

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

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

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

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 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.

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.

Do not use it in driving because of the danger of making the trailer skid.

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.

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.

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

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.

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.

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.

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.