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.
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
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,
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
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
for a brake application.
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,
to be released from the air reservoil to brake air
chambers to which are attached slack adjusters on