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

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.

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.

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.

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.

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.

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

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.

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

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.

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

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.

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

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.

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.

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.

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. 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.

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.

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.

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.

’s specification usually 20 – 45 psi.

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.

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.

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