Air Brake Manual

Tractor and Trailer Coupled
28
20
6
16
39
In this illustration, the trailer has been coupled to the
tractor, and the control (service) and supply (emergency)
lines have been coupled using glad hands
(20).
The trailer unit has a reservoir (16) installed. This
reservoir will provide a volume of air near the trailer
chambers for normal or emergency braking. The
trailer reservoir is fitted with a drain valve (6), the
same as a tractor reservoir.
A relay emergency valve (39) is mounted on the
trailer reservoir or to the trailer frame near the brake
chambers. The relay emergency valve serves three
main functions in the system:
1. It 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. It directs trailer reservoir air pressure to the
trailer brakes, causing an emergency application of
the brakes. This action will occur automatically in
the event of ruptured or parted air lines between
tractor and trailer, or loss of air pressure from the
power unit service system. A ruptured control
(service) line would not cause an emergency brake
application on the trailer until a brake application
was made, and this would cause a rapid loss of air
pressure from the system. The driver at any time
may operate the cab-mounted trailer supply valve
(28) to cause an emergency application of the trailer
brakes.
3. It has a one-way check valve which stops air
pressure in the reservoir from going back to the
source of the supply.
46