Welcome back to the second Part in our series taking a look at how you go about coupling and uncoupling your trailer. In our first Part, we took a look at the bare basics involved with getting the trailer and the tractor connected.

Today, we will dive into more detailed aspects of the process, from checking your trailer’s height to handling air supply. So without further delay, let’s dig in.

Checking the Trailer Height

When you check the trailer height, check it in relation to your tractor’s fifth wheel. Always make sure the trailer is low enough that it makes contact with the middle of the fifth wheel, but not so much that it puts any huge amount of pressure on it.

The trailer should be raised slightly as the tractor is backed under it. When a trailer is too low, it runs the risk of being struck and damaged. Conversely, if it is too high, it may not couple correctly, or it could miss the pin entirely and potentially strike the rear end of your cab.

To address this, raise or lower the trailer as needed through cranking the landing gear up or down. You can also raise or lower the fifth wheel using the tractor’s adjustable air suspension (if so equipped). Either way, when you reach the desired height, secure the landing gear crank.

The final step in the process should include checking to make sure the kingpin and the fifth wheel are both completely aligned. Also, ensure the jaws are open, especially if the truck has been moved away.

Air Line Connection

Before you can go about connecting the tractor to the trailer, you have to ensure your trailer brakes are in good working order. Consider that most modern trailers will already have the brakes set, but if your trailer was built prior to 1975, you may need to apply air to them to ensure they are set.

Even if the trailer is equipped with spring brakes, it’s a good idea to connect the air lines. This way you ensure your brakes are set across the board.

There are essentially two different air lines that would need to be connected, the “service” and “emergency” lines. You can distinguish between the two through a number of methods.

They could be color codes, with blue or black for the service line and red for emergency, they could be stamped with words, or the glad hands (rubber grommets) could be in a certain shape, with square for service and round for emergency. The emergency glad hand is also usually on the right side when you are facing the trailer.

Once you’ve identified the two lines, you want to check all four glad hand seals for any cracks or other damage. If none is found, proceed with connecting the tractor’s emergency air line with the trailer’s emergency glad hand. After you’ve ensured a good connection, engage the safety latch or other mechanism that keeps the lines sealed together. Once the emergency line is done, connect the service line the same way.

Also make sure the air lines are supported and aren’t in danger of being crushed or caught in something as the trailer is connected to the tractor. Finally, ensure there isn’t too little slack in the lines, which could cause the glad hands to uncouple.

Pre-1975 Trailer Air Supply

If you have determined you are using a trailer that do not have already preset brakes, there is an additional step you must take.

Once you are in the cab, with the engine off, push in the air supply knob. Or you may need to move the tractor protection valve control from the “emergency” to “normal” position. This will ensure air is delivered to the trailer brakes system.

Once the air pressure reports normal, check the pressure gauge or listen for any air loss. Apply and release the trailer brakes and ensure you can hear the sound of their operation.

With air supply covered, join us back here next week, when we finish out coupling and then move on to uncoupling. In the meantime, drive safe out there!