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2-4
G R O U P 2-BRAKES
DIAGNOSIS GUIDE—POWER BRAKES V A C U U M BOOSTER—DASH MOUNTED (Cont'd)
BRAKES DRAG
BRAKES GRAB
SELF APPLICATION OF
BRAKES WHEN ENGINE
STARTS
Sticking valve plunger.
Sticking actuating valve assembly.
Leak in rear housing.
pheric pressure into rear chamber.
Diaphragm out of locating radii
Sticking or unseated atmospheric
in housings and allowing atmos-
valve.
PRELIMINARY TESTS—POWER
BRAKES
With the engine stopped, eliminate
all vacuum
from
the system by
pumping
the brake pedal several
times. Then push the pedal d o w n as
far as i t will go, and note the effort
required to hold i t in this position. If
the pedal gradually moves down-
ward under this pressure, the hy-
draulic system is leaking and should
be checked by a hydraulic pressure
test.
With the brake pedal still pushed
down, start the engine. If the vac-
u u m system is operating properly,
the pedal will m o v e downward. If
the pedal position does not change,
the vacuum system is not operating
properly and should be checked by
a vacuum test.
V A C U U M TESTS
CHECK VALVE TEST
Disconnect the l i n e from the bot-
tom of the vacuum check valve, and
connect a vacuum gauge to the
valve. Start the engine, run i t at idle
speed, and check the reading on the
vacuum gauge.
T h e gauge should register 18-21
inches of vacuum. Stop the engine
and note the rate of vacuum drop.
If the vacuum drops more than one
inch in 15 seconds, the check valve
is leaking. If the vacuum reading
does not reach 18 inches or i s un-
steady, an engine tune-up is needed.
R e m o v e the gauge and reconnect
the vacuum line to the check valve.
BOOSTER TEST-BENDIX
PISTON TYPE P-350 A N D 4000
Disconnect the vacuum l i n e from
the booster end plate. Install a tee
fitting in the end plate, and connect
a vacuum gauge (No. 1) and the vac-
u u m line to the fitting. Install a sec-
ond vacuum gauge (No. 2) in place
of the pipe plug in the booster con-
trol valve body.
Start the engine, and note the vac-
u u m reading on both gauges. If both
gauges do not register manifold vac-
u u m , air is leaking into the vacuum
system. If both gauges register mani-
fold vacuum, stop the engine and
note the rate of vacuum drop on
both gauges. If the drop exceeds one
inch in 15 seconds on either gauge,
air is leaking into the vacuum sys-
tem. Tighten all vacuum connections
and repeat the test. If leakage still
exists, the leak m a y be localized as
follows:
1. Disconnect the vacuum line and
gauge N o . 1 from the booster.
2. Connect vacuum gauge No. 1
directly to the vacuum line. Start the
engine and note the gauge reading.
Stop the engine and check the rate of
vacuum drop. If gauge N o . 1 does
not register manifold vacuum, or i f
the vacuum drop exceeds 1 inch in
15 seconds, the leak is in the vacuum
line or check valve connections.
3. Reconnect vacuum gauge No.
1 and the vacuum line to the tee
fitting. Start the engine, and run it at
idle speed for one minute. Depress
the brake pedal sufficiently to cause
vacuum gauge N o . 2 to read from
zero to 1 inch of vacuum. Gauge
N o . 1 should register manifold vac-
u u m of 18-20 inches. If the drop of
vacuum on Gauge N o . 2 is slow, the
air cleaner, or air cleaner line, m a y
be plugged. Inspect and clean the
air cleaner if necessary.
4. Release the brake pedal and
observe the action of gauge N o . 2.
U p o n releasing the pedal, the vac-
u u m gauge must register increasing
vacuum
until manifold vacuum i s
reached. T h e rate of increase must
be smooth, with no lag or slowness
in the return to manifold vacuum. If
the gauge readings are not as out-
lined, the booster is not operating
properly and should be removed and
overhauled.
BOOSTER TEST-MIDLAND
D I A P H R A G M TYPE
P-500 A N D 5000
R e m o v e the pipe plug from the
rear half of the booster chamber, and
install a vacuum gauge. Start the
engine and run i t at idle speed. The
gauge should register 18-21 inches
of vacuum.
1. With the engine running, de-
press the brake pedal with enough
pressure to show a zero reading on
the vacuum gauge. Hold the pedal
in the applied position for one min-
ute. A n y downward movement of the
pedal during this time indicates a
brake fluid leak. A n y kickback (up-
ward movement) of the pedal indi-
cates brake fluid is leaking past the
hydraulic piston check valve.
2. With the engine running, push
d o w n on the brake pedal with suffi-
cient pressure to show a zero read-
ing on the vacuum gauge. Hold the
pedal down, and shut the engine off.
Maintain pedal position for one min-
ute. A kickback of the pedal indi-
cates a vacuum leak in the vacuum
check valve, in the vacuum line con-
nections, or in the booster.
HYDRAULIC PRESSURE TEST
Connect a 2000-psi hydraulic pres-
sure gauge to a bleeder screw open-
ing at one of the brake cylinders.
Bleed the air from the hydraulic sys-
tem at the point of attachment of the
gauge.
R e m o v e the pipe plug from the
rear of the booster body or the trailer
brake control line port, and connect
a vacuum gauge at this point. With
the engine running, apply the brakes
enough to obtain a zero reading on
the vacuum gauge. Then, note the
reading on the pressure gauge. T h e
m i n i m u m hydraulic pressure for each
type and size of vacuum booster is
given in Part 2-4. If the engine vac-
u u m
is higher or lower than 20
inches H g , the vacuum booster hy-
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