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the camshafts of each wheel brake mechanism. The
action of the air chambers causes the slack adjust-
ers to rotate the camshafts, forcing the brake shoes
into contact with the brake drums for a brake ap-
plication.
(3) Releasing semitrailer brakes. When the brake pedal
of the towing vehicle is released, a drop in pressure
in the service air line causes the emergency relay valve
to release the compressed air from the brake air cham-
bers.
This
release
of
pressure
permits
springs
in
the
air chambers and wheel brake mechanisms to pull the
brake
shoes
away
from
the
drums.
The
extent
of
brake release, like the amount of brake application,
is
in
direct
to
the
movement
of
the
brake
pedal on the towing vehicle.
c. Emergency Relay Valve Assembly. The
emergency
relay
valve assembly (fig. 12), on semitrailers M270 and M269, is
The  emergency  re-
mounted on the semitrailer air reservoir.
lay valve assembly (fig. 13), on semitrailer M270A1, is mounted
on a cross member of the chassis frame at a point above and
The  valve  assembly  performs  the  same
between the axles.
It  directly  controls  the  service
basic function on all vehicles.
brakes on the semitrailer. It speeds brake action by releasing
air
from
the
reservoir
on
the
semitrailer
directly
to
the
brake
air  chambers,  eliminating  the  loss  of  time  that  would  result
if  sufilcient  air  to  operate  the  brakes  had  to  travel  from  a
reservoir
on
the
towing
vehicle
to
the
semitrailer
brake
air
chambers. In addition, this valve assembly controls the flow
of air to and from the semitrailer air reservoir and its auto-
matically  applies  the  brakes  in  the  event  that  the  semitrailer
breaks away from the towing vehicle or there is a serious leak
in the emergency air line.
d. Service Air Line. The service air line (figs. 12 and 13)
on
the
semitrailer
extends
from
the
air
hose
coupling
(tagged
SERVICE)  on  the  right  side  of  the  front  cross  member,  along
the right chassis frame beam to an elbow in the top of the
emergency  relay  valve.  Its  purpose  is  to  transmit  changes  in
The changes
air pressure which cause this valve to function.
in pressure result from the brake control on the towing vehicle
being operated and the changes are dictated by the extent to
which
the
control
is
applied
or
released.
The
emergency
air
line
(figs.
e.
Emergency
Air
Line.
and
13)
on
the
semitrailer
extends
from
its
air
hose
coupling
(tagged
EMERGENCY),
along
the
inside
of
the
left
chassis
TAGO 1321B
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