A trailer breakaway (Fig. 45) would result in a separation of the service line and supply line. Sudden loss of air pressure in the supply line triggers the relay emergency valve, which causes the trailer reservoir to deliver its air directly to the trailer brake chambers. This places the trailer brakes into emergency application.

Loss of pressure in the supply line also causes the trailer-supply valve to automatically shift to the closed position.

The tractor brakes are operable, without air loss, because the tractor protection system has isolated the tractor.

The trailer brakes will remain applied until either the pressure in the trailer reservoir is drained off or the supply line is repaired and the system is recharged.