On a combination vehicle, what happens to the trailer if the air line between the tractor and trailer breaks while driving?

AIR LINE BREAK — TRAILER RESPONSE: Combination air brake systems have a fail-safe design. If the air supply line (emergency line) breaks or disconnects: air pressure in the trailer system drops rapidly; when it falls to a low level (around 20-45 psi), the trailer's SPRING (emergency) BRAKES apply automatically — spring brakes are held OFF by air pressure, so loss of air causes them to apply; the TRACTOR PROTECTION VALVE closes automatically to seal off the tractor's air system, preserving enough air for the tractor to stop safely; this prevents total loss of braking and stops the trailer; THE TWO AIR LINES: Service (control) line — carries the signal to apply brakes during normal braking; Emergency (supply) line — supplies air to charge the trailer reservoirs and hold off the spring brakes; if the supply line fails, spring brakes apply; this fail-safe design means a broken air line results in braking (stopping), NOT loss of brakes — a critical safety feature; drivers should understand this system and know the location of the trailer air supply valve in the cab.