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 properly connected to the service brake system of the towing
vehicle, the service brake pedal on the towing vehicle controls
To produce and maintain a con-
the brakes on both vehicles.
stant
supply
of
compressed
air
to
actuate
and
operate
the
semitrailer brakes, the towing vehicle is equipped with an air
compressor, reservoirs which store a supply of compressed air,
a governor for controlling the compression of air, an air gage
for showing the pressure of air in the system, a safety valve
to
insure
against
excessive
pressure,
and
air
lines,
air
hose
couplings, air hose, and shutoff cocks to convey compressed air
to the brake system on the semitrailer.
Note. Although these units are located on the towing vehicle, all must
operate effectively to insurc proper. performance of the semitrailer brakes.
b. Operation of Semitrailer Brakes.
(1) Filling air reservoir.
When the air brake hoses for
the service and emergency air lines are connected be-
tween the towing vehicle and the semitrailer and the
air shutoff cocks on the towing vehicle are opened,
air
flows
through
the
emergency
air
line,
air
cleaner
filter, and emergency relay valve to fill the air reser-
Air pressure is built up to
voir on the semitrailer.
equal the pressure in the compressed air system of
the towing vehicle.
(2) A p p l y i n g
semitrailer
brakes.
(a)
is applied to the brake pedal of the towing vehicle,
air pressure is directed through the service air line
to the emergency relay valve on the semitrailer.
Through connecting lines, the emergency relay valve
releases compressed air from the air reservoir to
each brake air chamber. A diaphragm and push rod
in each air chamber, when under air pressure, moves
Hy-
a
piston
in
each
hydraulic
master
cylinder.
draulic
fluid
present
in
the
body
of
each
master
cylinder
is
forced
into
the
wheel
brake
cylinders
for a brake application.
(b)
Semitrailers
M270
and
pressure is applied to the foot pedal of the towing
vehicle, air pressure is directed through the service
air line to the emergency relay valve mounted on and
connected to the air reservoir on the semitrailer,
The
emergency
relay
valve
causes
compressed
air
to be released from the air reservoil to brake air
chambers to which are attached slack adjusters on
TAGO 1321B
20
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