In vehicles equipped with an automatic tractor protection valve, the valve will pop out if air pressure drops to a level between 20 and 45 psi. When the valve pops out, the protection valve will close, stopping the flow of air.

The earliest and best way to recognize the beginnings of a trailer skid is by seeing the trailer veer off-course in your mirrors.

Combination vehicles need more space on the road than other commercial vehicles because they are longer and need more space to turn and stop. It is especially important to properly manage space when you are operating a combination vehicle.

Tractor protection controls in older vehicles may be operated by levers instead of knobs. If an air supply control is set in its "emergency" position, the air supply will be stopped and the trailer emergency brakes will be applied.

When operating a vehicle with an Anti-Lock Braking System (ABS), you should apply your brakes in the same manner as you would if operating a vehicle without ABS.

Tractor protection controls in older vehicles may be operated by levers instead of knobs. If an air supply control is set in its "normal" position, it is properly set for you to pull a trailer.

Before a trip, ensure that air in the air brake system reaches all trailers. Do this by waiting for air pressure to build, then sending air to both the emergency and service lines and opening the shut-off valves on the rear of the last trailer. If air escapes from the shut-off valves in the rear of the combination, the air is being supplied to the entire vehicle.

Correctly following the steps to couple and uncouple trailers is vital to safely operating a combination vehicle. Before backing your tractor under the trailer while coupling, make sure your trailer brakes are locked.

Double and triple combinations are the most vulnerable to turning over as a result of the "crack-the-whip" effect.

Trailers can overturn as a result of the "crack-the-whip" effect. The last trailer in a combination is the most prone to tip over due to this effect. Drivers should be aware of the rearward amplification of their specific vehicles and drive accordingly.

Before a trip, be sure to inspect all couplings. There should be no space between the upper and lower fifth wheel.

Rollovers can happen when a combination vehicle is turned too quickly. To help prevent the risk of rollover, it is important to keep the vehicle's center of gravity low by loading cargo as close to the ground as possible.

Combination vehicles are usually heavier and longer than single combination vehicles. Operating a combination vehicle requires a higher level of driving skill than operating a single commercial vehicle.

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

Never use the trailer hand valve while operating your vehicle. Using the trailer hand valve while driving can cause your trailer to skid.

Push in the trailer air supply control to supply the trailer with air. Pull out the trailer air supply control to turn off the air supply and activate the emergency brakes.

When starting a trip, be sure that all shut-off valves are in the open position, except for the last valves on the rear trailer. It is important that air reaches the brakes on all trailers and that the air is not able to escape from the back of the vehicle.

At speeds below 40 mph, you should allow at least one second of following distance for every 10 feet of your vehicle. Follow the same formula when traveling at faster speeds, then add one additional second to your following distance. Remember that larger vehicles require more space to stop than smaller vehicles.

When backing up to couple a trailer, you should position the tractor directly in front of the trailer. Trying to couple while backing at an angle could cause the trailer to move and the landing gear to break.

In a combination vehicle with air brakes, the tractor protection valve keeps air in the tractor or truck brake system in the event of the trailer breaking away or developing a leak.