Rupture of the supply line (or an uncoupling of the supply line glad hands - Fig. 47) results in a pressure drop in the supply line between the trailer-supply valve and relay emergency valve. This triggers the emergency action of the relay emergency valve, placing the trailer brakes into emergency application. As in the previous examples, the trailer-supply valve will shift to the closed position.
Operation of the tractor brakes will not be affected if the tractor protection system is in working condition.
The relay emergency valve must be of the “no-bleed-back” type, so no air is lost from the trailer.
Note: Depending on the type of tractor protection system used, air loss from the tractor will stop immediately or it will bleed down to a minimum of 20 psi (138 kPa) and then shut off. Most newer units will shut off much higher than 20 psi.