Chevrolet 70 1969 Series, Shop Manual

Page: 235 / 913

BRAKES 5B-42
chambers). Of these three units the actuator is
used by either the service or parking systems,
while the other units are used wholly by the park­
ing system to operate the foundation brake. These
three units are used in conjunction with a standard
foot control valve and a slack adjuster connection
to standard cam-operated wheel brakes.
Since this D D 3 system serves both parking
and service brake systems, information on m e­
chanical parking brake in sub-section for "P A R K ­
ING B R A K E " applies to those vehicles which do
not have D D 3 parking brake.
The name of the system is derived from the
description of the actuator (chamber). " D D " de­
notes the double diaphragm and the suffix " 3 "
denotes the triple action for service, parking and
emergency braking. The actuator functions nor­
mally as a service brake chamber but in addition
has a means of mechanically locking a brake ap­
plication so it can be used for parking. The DD3
brake actuator is used only at the rear wheels.
Standard air-operated chambers are used at the
front wheels.
S E R V IC E B R AKE O P E R A T IO N
Figure 16 shows schematically how the sys­
tem works. With the handle of the push-pull con­
trol valve pushed in, air pressure from the park­
ing reservoir is delivered through the push-pull
control valve to the control port of the inversion
valve. From there it is routed to the lock port of
the D D 3 brake actuator. Air pressure, acting on the
actuator piston moves it forward to contact the
rollers which roll up the ramp of the piston hold­
ing them away from the push plate shaft. This al­
lows the push plate and shaft to move freely in
both directions for normal service brake applica­
tion and release.
PARKING O P E R A T IO N
To park, the handle of the push-pull control
valve is pulled out. This action closes the inlet
valve, closing off any further air supply to the
push-pull valve control port (and from there
through the inversion valve to the DD3 brake ac­
tuator lock port). At the same time, this "pulling
out" action opens the exhaust valve of the push-
pull control valve, which allows the lock port of
the DD3 brake actuator to vent through the control
port of the inversion valve and then through the
push-pull control valve and the open exhaust port
of the foot control valve.
When the D D 3 brake actuator lock port is
vented the roller spring forces the rollers against
the ramp on the collar to engage them with the
push plate shaft.
When the control port of the inversion valve
is vented, the inversion valve piston moves for­
ward and opens the inlet exhaust valve. This opens
the line from the parking reservoir and allows air
pressure to flow into the parking port of the D D3
brake actuator. Full parking reservoir pressure is
delivered and a parking brake application results.
With the rollers against the shaft, the shaft can
move forward but is locked so that it cannot return
for release. To release parking brake application
it is necessary to have full pressure in the air
system, "push in" on push-pull control valve (make
sure it stays "in") and make a full 100 psi service
brake application.
E M E R G E N C Y O P E R A T IO N
The vehicle is equipped with an air pressure
gauge (in the instrument cluster) and (as optional
equipment) a low air pressure buzzer. When the
operator is aware of a loss of air pressure as
indicated by the gauge or the buzzer, and the
service brakes will not stop the vehicle, a manual
emergency application of the parking brake should
be made by pulling out the handle of the push-pull
control valve. Brakes then operate as described
under the heading of "Parking Operation.”
In the event the operator does not respond to
the warning system or if the system fails to func­
tion, or if the loss of air pressure is too sudden
for action, and if the air pressure in the parking
reservoir falls below 40 psi, the push-pull control
valve will automatically "pop" out, causing pres­
sure in the D D 3 actuator lock port to exhaust,
which will result in an automatic parking (emer­
gency) brake application.
DD3 BRAKE ACTUATOR
OPERATION
N O R M A L R U N N IN G
Through the push-pull control valve and an
inversion valve, air enters the actuator locking
port and exerts pressure on the locking piston
grommet. The resultant force moves the locking
piston forward against the rollers and roller
spring. The beveled, or ramp, end of the piston
will pick up and hold the rollers away from the
shaft. As long as air pressure remains against the
locking piston grommet and the rollers are not in
contact with the shaft, normal service brake ap­
plications will permit the shaft to move freely,
back and forth, past the locking mechanism. When
a normal service brake application is made, air
enters the actuator service port and applies pres­
sure against the service diaphragm. The diaphragm
moves the push plate and shaft out, applying the
CHEVROLET SERIES 70-80 H EAVY DUTY TRUCK S H O P M A N U A L