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return spring
assures complete extension of the
bellows
whenever the brakes are not being ap-
plied.
In the released position, the bellows is filled
with air (atmosphere).
The power produced by
the unit
is developed when vacuum removes the
air from
the bellows and causes the bellows to
contract
(due to continued atmospheric pres-
sure on the outside of
the bellows).
When application
of the brakes begins, air-
vacuum
power
developed
in
the power
unit
is
applied through
the
shaft
and
yoke
assembly
to the brake
pedal.
I
n this
way, contraction
of
the bellows
assists
in
applying the brakes.
The design of the
unit is
such
that
even
though the
power unit performs much of the
work necessary
to
apply
the brakes, the oper-
ator's
control
of
the braking
process is complete
and he loses none of the
brake
"feel.
"
Although
there
is
a mechanical contact be-
tween the
power unit and the brake
linkage
through the
push rod and
yoke
assembly,
this
contact exists only when the
power unit is
assisting
in a brake
application.
Therefore, if
there
is
a
loss of vacuum, as with a stalled en-
VACUUM VALVE"
CLOSED
AIR VALVE
OP
"
N'~
gine, the brake pedal is free to move,
com-
pletely
independ
ent of the power unit.
OPERATION
Released
Position
In
this position the air
valve
is open
and
air
(atmosphere)
is freely admitled to the
air-
vacuum
bellows (Fig. 13). The
vacuum
valve is
closed, preventing evacuation
of air
from the
bellows. The air valve spring pressure tends to
close
the air valve. but the brake pedal pull-
back
spring
pressure
(transmitted through
linkage to
the
valve
operating
r
od)
over
comes
the air valve
spring
pressure and tends
to
keep
the
air valve in an
open (released)
position,
thereby
maintaining atmospheric
pressure
in-
side as well as
outside
the
air-vacuum
bellows.
Applied
Position
When pressure is
applied
to
the brake pedal,
restraint
on
the
air
valve spring is removed and
the air valve is closed by
spring
pressure (Fig.
14).
Continued
movement
of
the
air
valve, due
to air valve spring pressure, will overcome
the
vacuum valve
spring pressure
and open the
/,,",'.YAC'UU'"
BELLOWS
VACUUM
56
x
30
Fig.
13 -
Power Unit
in " Releored"
Position
14
-
---
4
I
I
•
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