In the diagram above, the trailer has been coupled to the tractor and the service and supply lines of the units have been coupled by using glad hands.

The trailer unit has a reservoir (2) installed. This tank will provide a volume of air near the trailer chambers for normal or emergency braking. The tank is equipped with a draincock.

A RELAY EMERGENCY VALVE (25) is mounted on the trailer reservoir. This valve can also be mounted directly on the trailer frame near the brake chambers. The relay emergency valve serves three main functions in the system:

1. The RELAY part of the valve relays air from the trailer reservoir to the trailer brake chambers during a brake application. This part of the valve operates like the relay valve previously discussed. It also provides a quick release of the trailer brakes.
2. The EMERGENCY part of the valve directs trailer reservoir pressure to the trailer brakes, causing an emergency application (dynamiting). This action will occur automatically in the event of ruptured or parted air lines between tractor and trailer, or loss of air from the main reservoir system. The driver may operate the cab-mounted trailer supply valve (24) to cause an emergency application of the trailer brakes.
3. The relay emergency valve has a one-way check valve that stops air in the reservoir from going back to the source of the supply.