Red is for parking...

Red and Blue airlines connect the tractor air brake system to the trailer.

Red is emergency brakes and applies system air pressure to the Spring (Parking) Brakes on the trailer when the red valve in the dash is depressed. This air pressure releases the parking brakes, allowing the trailer to move. If this line were to fail, the spring brakes on the trailer would automatically engage, locking the tires.

Blue is service brakes. When the driver depresses the foot brake in the cab, or pulls the hand valve (Johnny Bar), a proportional amount of air is directed through the line to energize the service brakes on the trailer. The harder the pedal is depressed, or the more the bar is pulled, increases air pressure to the trailer service brakes.

A tractor protection valve, located at the rear of the cab or sleeper, maintains tractor air pressure, regardless of the red or blue airline condition, ensuring the tractor always has brakes.

If the air lines were switched (and it is possible), the spring brakes on the trailer would remain engaged (no air pressure to release) and would not allow the tires to roll.

The yellow valve in the dash releases the tractor spring brakes.