Chevrolet 10 Series 1968, Service Manual, Page: 343 / 713

BRAKES 5-100
Leakage Tests
Apply suds solution to supply and delivery ports and
around cover and cap while performing the above tests.
No leakage should be evident.
DD2 SAFETY ACTUATOR (Fig. 144)
Disconnect and Plug Line to Emergency Port of Actu­
ator. With the actuator in the released position, make
several foot brake applications and note that actuators
apply and release properly. Check push rod travel to be
sure it is at a minimum without brakes dragging.
Reconnect Emergency Line to Actuator. With the brake
system at maximum pressure operate the Emergency
Control Valve handle in and out several times. Note that
actuators apply and release properly.
Leakage Tests
With full system pressure application on the service
diaphragm, apply soap solution around the clamping ring.
No leakage is permitted. With a full system pressure
application on the emergency diaphragm, repeat above
procedure.
If leakage is detected around the clamping ring, the
bolts should be tightened only enough to stop leakage.
Over-tightening could distort the clamping ring.
DOUBLE CHECK VALVES (Fig. 145)
System Checking
Double check valves are used in different combina­
tions on the various installations. To test their opera­
tion, operate the brake system through its various modes
of operation (refer to preceding text). If air pressure
is being routed correctly at delivery pressure, the
Double Check Valves are functioning correctly. If one
of the modes is not correct (all other valves operating
correctly) the shuttle of the affected valve is stuck. If
all modes operate correctly but delivery pressure to the
COMMON DELIVERY
OR
PLUGGED
COMMON
DELIVERY
F ig . 145-Double Ch eck V a lv e Components
brake chambers is noticeably lower than supply pres­
sure, there is air leakage past the shuttle. In either
case, the valve should be removed and cleaned or re ­
placed depending upon the malfunction.
Leakage Tests
Apply bubble solution at all connectors and around
cover. Operate brake system through its modes noting
any external leakage. Test for internal leaks (past
shuttle) as described under "Testing".
MANUAL AND AUTOMATIC EMERGENCY
CONTROL VALVE (Fig. 146)
System Checking
Start test with no air in system, and note upon initial
build-up that a brake application is made by the DD2
Actuators* At approximately 65 psi brakes should re ­
lease, and system will continue to build-up to governor
cut-out. Turn off engine.
With system fully charged make several service brake
applications and note that at approximately 45 psi DD2
Actuators will automatically apply brakes and remain
applied until service reservoir pressure is restored to
over 60 psi.
Leakage Tests
Start leakage checks with system pressure up to
governor cut-out and brakes released.
Check exhaust port for possible leakage at: (1) Large
piston grommet, (2) Small piston grommet, (3) The in­
let valve or its seat, and (4) Exhaust valve or its seat.
CHEVROLET TRUCK SERVICE MANUAL