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Preliminary Service Information
9
Passage Open to Atmosphare
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Pin ot Pront of Bushing
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Passage Closed
vacuum
Fig.
13.
Further
pedal
movement
has moved
the
valve
back
far enough
to uncover
again
the passage
in
the
operating
tube keeping
closed
the passage
between
the
two chambers,
and thus opening
the front
chamber
to
the
atmosphere
Fig.
15.
The pedal
movement
has ceased,
allowing
the
valve
to close
the
atmospheric
passage
automatically.
As the air in the front
chamber
cannot
escape,
it
pre-
vents
the piston
from
moving
forward
and
keeps
the
piston
in the position
just
reached
The pedal in Fig. 12 has been depressed only enough
to push the valve rod and valve back to a point closing
the vacuum passage between the two cylinder cham-
bers. A slight additional pedal movement has pushed
the valve rod and valve back the rest of the way in
Fig. 13, admitting air into the front chamber. At-
mospheric pressure then forces the piston back as Jong
as the pedal movement continues.
In Fig. 14, atmospheric pressure on the front side
of the piston is forcing the piston back as indicated
by the arrow to assist the pedal in making application
of the brakes. The pedal is still moving downward,
keeping the atmospheric passage open.
In Fig. 15 the operator has applied the desired
braking force and the pedal movement has ceased,
allowing the valve to close the vacuum and at-
mospheric passages automatically. With the vacuum
and atmospheric passages closed, the assister ceases to
Fig.
14.
Atmospheric
pressure
in the front
chamber
is
forcing
the piston
back as indicated
by the large
arrow.
The pedal
is still
moving
downward
so as to keep
the
valve back,
leaving
the atmospheric
passage
open
build up any additional force while the air already in
the assister helps to hold the piston and pedal in
place.
Any additional pedal movement forward will
push the valve rod and valve back, opening the
atmospheric passage and in turn forcing the piston
back to apply the brakes still further.
In Fig. 16 the operator has removed his foot from
the pedal, allowing the valve rod and valve to be
moved all the way forward by the action of the valve
spring. This forward motion of the valve has opened
the vacuum passage between the two cylinder cham-
bers, equalizing the vacuum on both sides of the
piston and allowing the brake retracting springs to
pull the pedal back and the piston forward to their
normal released position.
Adjustments: The brake assister and pedal adjust-
ments are made in the same manner as the assister
adjustments on the 370-B and, 452-B/ cars.
The
Fig.
16.
The pedal
has been
released,
permitting
the
valve
to
return
to its
forward
position
opening
the
passage
between
the
two
chambers.
This
equalizes
the vacuum
pressure
on both
sides of the piston
allow-
ing
the
piston
and
pedal
to return
to
their
normal
released
positions
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