How to Connect Air Lines to a Trailer

Connecting the tractor-trailer air lines requires careful observation.

Big rigs, or tractor-trailer vehicles, have air lines that join the tow tractor to the trailer, or from the trailer to the dolly, and then on to a second trailer, if so equipped. The foot brake or trailer handbrake controls the "service" line. The service line uses a relay valve to regulate pressure to the trailer brakes. The "emergency" air line supplies air to the trailer tanks, and activates the emergency brakes to the combination vehicle. Both air lines have couplers called "glad hands" which form a locking seal between the lines. Connecting the lines requires a few steps.

Things You'll Need

• Wheel chocks
• Rags

Instructions

1. Chock the wheels of the trailer, or trailers, if equipped with a double setup. Back the tractor up slowly in a straight line until the fifth wheel touches the trailer. Secure the parking brake and put the transmission in neutral. Check the alignment and trailer height, particularly the king pin and fifth wheel. Set the trailer height slightly lower, by turning the landing gear crank, so the tractor coupler will wedge underneath it.

2. Unhook the (red-colored) emergency line coupler from its holder and check the glad hand grommet seal for cracks and dirt. Wipe it clean with a rag. Press the two seals together while holding the glad hands at 90-degree angles to each other. Turn one glad hand against the other with a twisting motion to join and lock the coupler into position.

3. Unhook the (blue-colored) service line from its holder and check the glad hand grommet seal for cracks and dirt. Wipe it clean with a rag. Press the two seals together while holding the glad hands at 90-degree angles to each other. Turn one glad hand against the other with a twisting motion to join and lock the coupler into position. Make sure both lines do not hang down between any metal surfaces and that they will not become pinched when the tractor locks to the trailer.

4. Man the cab. Push the air knob inward, or move the emergency selector knob to the "Normal" position. Check the air pressure gauge for a normal reading. Shut the engine off and activate the foot brake. Listen for the pressure release and trailer movement, to confirm the connection. Start the engine and place the transmission in its lowest reverse gear. Pull the air supply knob valve open, or move the air supply knob to the "emergency" position. This sets the trailer brakes. Back slowly until you feel the coupling jolt.

5. Release the emergency brake and put the tractor in its lowest forward gear. Drive forward just enough to feel the coupling pressure exerted on the trailer. Shut your engine off until you need to leave. Remember to deactivate the emergency trailer brakes before driving off.

References

• Integrated Publishing: Trailer Air Lines
• LowestPriceTrafficSchool.com: Coupling Tractor-Semitrailers