Chevrolet 70 1969 Series, Shop Manual

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BRAKES 5A-15
VACUUM RESERVE TANK
A vacuum reserve tank is used as optional
equipment on all models using vacuum assisted
hydraulic brakes in series 70. This tank has a
minimum capacity of 1000 cubic inches and is
installed in the vacuum line between the vacuum
check valve and the power cylinder.
On conventional cab models, the tank is in­
stalled immediately below the left-hand door, be­
hind the cab step along with the vacuum booster.
On tilt cab models, the tank is installed be­
hind the engine, in front of the rear axle, and be­
tween frame side rails.
To replace tank, disconnect lines and brackets
connecting tank to vehicle.
VACUUM GAUGE
There are several vacuum gauges used on
vehicles covered by this manual. All operate on
the same principle and all are factory set and
sealed units which are not adjustable or repair­
able. If a gauge fails to operate or operates im­
properly, it must be replaced. Gauges differ in
size, appearance and color dependent upon truck
model. All are located in the instrument panel,
some as units in a cluster and some as separate
installations.
A vacuum gauge is standard equipment on all
vehicles in 70 series with Diesel engines and
vacuum power hydraulic brakes. A vacuum gauge
is optional equipment on all vehicles in series 70
with gasoline engines and vacuum power hydraulic
brakes.
If a vacuum gauge is suspected of operating
improperly it may be checked by comparing read­
ings with a test gauge which is known to be accur­
ate. Observe readings on vehicle gauge at engine
idle speed and at various, specific engine R PM up
to maximum. Install test gauge in convenient lo­
cation in vacuum line and observe readings on test
gauge at same, specific engine RPM. Manufacturer
specifications permit a variation of i 1 inch of
mercury at 5 inches and - 2 inches of mercury at
20 inches. Any variation beyond these limits is an
indication that the vehicle vacuum gauge should be
replaced. Before condemning a gauge which does
not register, or registers improperly, make cer­
tain that all vacuum lines in the system are free
of dirt and/or kinks and that all connections are
tight. System leakage can result in registration on
gauge which is not normal. Also, on models with
Diesel engine, make certain that vacuum pump is
properly installed and operating.
TYPE “F” FRONT BRAKE
(Refer to Figure 14)
Two identical brake shoes are arranged on
backing plate so that their toes are diagonally
opposite. Two single-end wheel cylinders are
arranged so that each cylinder is mounted between
the toe of one shoe and the heel of the other. The
two wheel cylinder pistons apply an equal amount
of force to the toe of each shoe. Each cylinder
casting is shaped to provide an anchor block for
the brake shoe heel.
Each shoe is adjusted by means of an eccen­
tric cam which contacts a pin pressed into brake
shoe web. Each cam is attached to the backing
plate by a cam and shoe guide stud which protrudes
through a slot in the shoe web and in conjunction
with flat washers and C-washers, also serves as
a shoe hold-down. Two return springs are connec­
ted between the shoes, one at each toe and heel.
With vehicle moving forward, both shoes are
forward acting (primary shoes), self-energizing
in forward direction of drum rotation. With ve­
hicle in reverse, both shoes are reverse acting
since neither is self-energized in the reverse
direction of drum rotation.
CHEVROLET SERIES 70-80 H EAVY DUTY TRUCK SH O P M A N U A L