BC Transit – Air Brake Course

Service Line Rupture (continued)
1) Because the air to apply the trailer brakes is taken from the supply line by the relay
function of the tractor protection valve, this causes an immediate reduction in pressure
in the supply line.
2) The emergency relay valve “senses” this loss of pressure and interprets it as a severed
supply line, and so, dynamites the trailer brakes. The one-way check valve in the
emergency relay valve closes to protect the remaining air in the trailer reservoir.
Air loss from service line
upon service brake
application
Left click to start animation
157