g. Service Air Line. The service
air line on the semitrailer extends from the
air hose coupling (tagged SERVICE) on
the right side of the semitrailer to the
relay valve. Its purpose is to transmit
multi-function valve to the emergency
changes in air pressure originated in
the towing vehicle, which causes the emergency
relay valve to function.
h. Emergency Air Line. The emergency air line on the semitrailer extends from
the--air hose coupling (tagged EMERGENCY), on the left side of the semitrailer to the
multi-function valve to the emergency relay valve. This air line transmits
compressed air to fill the semitrailer air reservoirs and maintain proper air
pressure, under control of the multi-function valve and the emergency relay valve,
to apply the brakes on the semitrailer.
i. Emergency Relay Valve. The emergency relay valve (1, fig. 4-24) is mounted
on the semitrailer frame. The emergency relay valve directly controls the service
brakes on the semitrailer. It speeds brake action by releasing air from the air
reservoirs, on the semitrailer, directly to the brake chambers. This valve controls
the flow of air to and from the semitrailer air reservoirs and automatically applies
the brakes if the semitrailer breaks away from the towing vehicle or if there is a
serious leak in the emergency air line.
Emergency Relay Valve (M871Al)
The multi-function valve (1, fig. 4-25) is mounted on
j. Multi-Function Valve.
the semitrailer frame and controls the brake chambers by utilizing both air
reservoirs for normal service braking, but reserving sufficient air pressure to
provide the required spring brake chamber release in the event of a system failure.