Vacu Um Power Cylinders - GMC 4000 Series Service Manual

Sec. 5A
Page 180
HYDRAULIC BRAKES
Vacuum power cylinders are used as standard
equipment or are available as optional equipment
on all vehicles with hydraulic brakes covered by
this manual.
All the power cylinders used on these vehicles,
both remote mounted and dash mounted (integral
with master cylinder) are the type used in the "one
to one" hydraulic system . That is, the hydraulic
fluid volume output is equal to input.
The power cylinder is a combined vacuum-
hydraulic power unit, utilizing vacuum and atm os­
pheric pressure for its operation.
The vacuum source on gasoline engine models
is the engine intake manifold. The vacuum source
on Toro-Flow D iesel Models is the vacuum pump.
The combination of vacuum in front of the
diaphragm and atmospheric pressure behind the
diaphragm results in the power application of the
brakes.
V A CU U M POWER BRAKE
SYSTEM TESTS
VACUUM TEST
With engine stopped, hand brake applied and
transm ission in neutral, apply brakes several
tim es to destroy all vacuum in system .
Depress brake pedal, and while holding foot
pressure on pedal, start engine. If vacuum system
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FRO M M ASTER
C Y LIN D E R
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c y l i n d e r
[C O N T R O L
! V A L V E
FRO M V A C U U M
S O U R C E
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• * . SU P P O R T
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B R A C K E T
CO N V EN TIO N AL MODELS
G M C S E R V I C E M A N U A L

VACU UM POWER CYLINDERS

' *
T O W H E E LS
FRO M AIR
FILTER
(A TM O SPH E R E )
V A C U U M
C O N N E C T IN G
TUBE
Figure 19— Pow er Cylinder Installed
is operating, pedal will tend to fall away under foot
pressure when engine starts, and le ss pressure
will be required to hold pedal in applied position.
If no action is felt, vacuum system is not func­
tioning.
Inspect vacuum lines for leakage, and for re­
striction caused by bent or kinked tube or hose.
If no fault is found in lines, trouble is in power
cylinder control valve, necessitating overhaul of
power cylinder.
HYDRAULIC TEST
Stop engine and again destroy all vacuum in
system . Depress brake pedal and hold foot p res­
sure on pedal. If brake pedal gradually falls away
under foot pressure hydraulic system is leaking,
either internally or externally.
Inspect all hydraulic line connections for leak­
age and make the necessary repairs. If no external
leaks are evident, inspect master cylinder and
wheel cylinders and replace parts as necessary.
If the condition still exists, an internal leak in
power cylinder is indicated, necessitating over­
haul of power cylinder.
POWER CYLINDER REPLACEMENT
The installed location of vacuum power cyl­
inders varies by model.
W PO W ER
■
C Y LIN D E R
/ / FRO M V A C U U M
FROM M ASTER / /
C Y LIN D E R
BLEEDER
V A L V E C O N T R O L
3 V A L V E
TO W H EEL . . ^
C Y U N D E B S
TILT CAB MODELS
FILTER)
S O U R C E
I L I A T M O SP H ER E
mm
lin e
V A C U U M
A
C O N N E C T IN G
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TUBE
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