Adjust the air supply knob to send air to the trailer brakes. Turn off the engine once the air pressure normalizes. Then listen for the sound of escaping air as the brakes are released.
After coupling, make sure that the air lines are not crossed. To do this, send air to the trailer brakes by adjusting the "air supply" knob to normal. Let the pressure normalize. Then turn off the engine and test the brakes. You should see and hear the brakes in action and then see the air pressure normalize again.
B Turn off the engine and listen for the release of air as you apply the brakes.
C Adjust the air supply knob to send air to the trailer brakes and check if the air pressure normalizes.