Chevrolet 10 Series 1968, Руководство по обслуживанию

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BRAKES 5-76
EMERGENCY AIR BRAKES (DD3)
INDEX
Page
General Description .............................................................. 5-76
Operation T e st and Leakage Tests ................................ .......5-76
Push-Pull Control Valve ............................................. .......5-76
Low P ressu re S w itc h ..............................................................
5_ 76
Inversion V alve ............................................................. .......5-76
DD3 Safety A c t u a t o r ..............................................................5-77
Page
Service Operations .......................................................... ....... 5-80
Push-Pull Control V alve ............................................. ....... 5-80
Low P ressu re Switch ................................................... ....... 5-81
Inversion V alve ............................................................. ....... 5-81
DD3 Safety A c t u a t o r .............................................................. 5-83
GENERAL DESCRIPTION
For 1968 an emergency air brake option is available
for T ilt Cab models only. The option includes a push-
pull control valve, low pressure switch, inversion valve,
DD3 safety actuators, air tank and connecting lines
(fig. 114).
The purpose of the system is to automatically and
safely apply the brakes in case of a leak in the emergency
brake system as well as warning the driver of the loss of
air. This is accomplished by the push pull control valve
automatically tripping to the on position when the emer­
gency brake system air pressure drops below approxi­
mately 40 psi. This sends air from the emergency tank
to the rear chamber of the DD3 (double diaphragm triple
action) safety actuators applying the brakes. At the same
low pressure the inversion valve will exhaust the air
from the parking port of the DD3 actuator. This will
cause the rollers in the DD3 unit to come in contact with
the shaft and cause a wedging action which will hold the
brakes in the applied position until the air is replenished
in the system.
OPERATION TEST AND LEAKAGE TESTS
PUSH-PULL CONTROL VALVE
O perating Test
1. With the air brake system pressure at zero, the
actuators should be applied (push rod fully extended).
2. Build up system pressure and hold control valve
plunger "in". The control valve plunger should re ­
main "in" when the system pressure is above 50 psi.
The brake actuators should release (push rod fully
retracted) after making a 100 psi service brake
application.
3. With the Truck reservoirs charged as above and with
the control valve plunger "in", pull the valve plunger
"out". The brake actuators should apply.
4. Push control valve plunger "in". The brake actua­
tors should release with a service brake application.
5. Make a series of brake valve applications to drop
system pressure and observe that the brake actua­
tors automatically apply when the pressure drops to
about 40 psi. Also note that the control valve plunger
comes "out".
LOW PRESSURE SWITCH
Operating Test
1. Operation of the Low Pressure Indicator may be
checked by reducing the reservoir pressure and
being sure that the contacts close when the reservoir
pressure is between 66 pounds maximum and 54
pounds minimum. The contacts will be closed when
the warning light or electrical buzzer operate.
INVERSION VALVE
Operating Test
1. With the air brake system built up to governor cut
out pressure and the brakes released, operate the
control valve to apply the brakes. Note that the
brakes apply.
2. Operate the control valve to release the brake appli­
cation and note that the delivered air from the inver­
sion valve is exhausted out its exhaust port. The
operation of the control valve will not necessarily
complete the release of the brakes. A 100 psi serv­
ice brake application after the control valve is
actuated should complete the release.
Leakage Test
NOTE:
Start leakage checks with system pres­
sure up to governor cut out and brakes released.
1. Check the inversion valve exhaust port for possible
leakage at (1) the large piston grommet; (2) piston
stem grommet or; (3) the inlet valve or its seat.
Slight leakage is permissible. While the inversion
valve is still in this position, the cap nut should be
checked for leakage by the seal ring.
2. Actuate control valve, applying the brakes, then
check the inversion valve exhaust port for exhaust
valve or seat leakage.
3. If the inversion valve does not function as described
or leakage is excessive, it can be repaired by follow­
ing the procedure outlined under "Service Opera­
tions" of this section.
CHEVROLET TRUCK SERVICE MANUAL