What is the purpose of the two air lines connecting the tractor and trailer (service and emergency lines)?

THE TWO-LINE AIR SYSTEM connecting tractor and trailer provides both service braking control and emergency protection. SERVICE LINE (also called CONTROL LINE): Color code is typically BLUE (check your specific vehicle — some use RED for service); when the driver presses the foot brake, the treadle valve sends a proportional pressure signal through the service line to the trailer's relay valve; the relay valve uses local trailer air tank pressure to apply the trailer's service brakes in proportion to the signal; releasing the foot brake vents the service line, releasing trailer service brakes; EMERGENCY LINE (also called SUPPLY LINE): Color typically RED; carries continuous high pressure (approximately 90-120 psi) from the tractor's supply system to the trailer's emergency relay valve and to the trailer's air tanks; this pressure HOLDS the trailer's spring brakes in the released (off) position; EMERGENCY/DISCONNECTION PROTECTION: If the emergency line is deliberately disconnected, kinked, or loses pressure (e.g., if the trailer breaks away from the tractor), the spring brakes automatically apply — stopping the runaway trailer; this fail-safe design means a separated trailer will not run free indefinitely. CONNECTION COLOR CODES: Glad hands (the metal connectors) are also color-coded to prevent cross-connection; connecting lines backward can result in loss of trailer braking control and is a pre-trip inspection error that should be caught.