GMC 4000 Series Service Manual page 233

Sec. 5B
Page 226
AIR BRAKES
brakes applied, coat the exhaust ports of the two-
way valve and limiting quick release valve with
soap suds. If leakage is excessive, replace valve.
b. Place handle of two-way valve in "SLIP­
PERY ROAD" position and with truck brakes ap­
plied, coat exhaust port of two-way valve with soap
suds. If leakage is excessive, replace valve.
TWO-WAY VALVE REPLACEMENT
IMPORTANT: Before disconnecting air lines,
either scribe a line or put a daub of paint on one
air line and corresponding port on valve. It is pos­
sible to connect the air lines to this valve in the
opposite position, with the result that the brake
application p ressures would be opposite to that in­
tended; that is, with valve knob in "SLIPPERY
ROAD" position, brake application would be for
"DRY ROAD" conditions and vice versa.
Removal
1. Block vehicle wheels.
2. Drain air pressure from brake system .
3. Disconnect air lines.
4. On tilt cab models remove screw s and re­
move plate and valve from instrument panel.
5. On other models, disconnect rodfrom lever
and remove bolts and valve from bracket.
DESCRIPTION
The DD3 brake actuator system consists of
three major units connected by air lines. These
are, the brake actuator (chamber), a push-pull
control valve (for manual parking and emergency
application), and an inversion valve (to direct the
flow of air pressure to either parking or locking
chambers). Of these three units the actuator is
used by either the service or parking system s,
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 DD3 system serves both parking and
service brake system s, information on mechanical
parking brake in sub-section for "PARKING BRAKE"
applies to those vehicles which do not have DD3
parking brake.
The name of the system is derived from the
description of the actuator (chamber). "DD" de­
notes the double diaphragm and the suffix "3" de­
notes the triple action for service, parking and
emergency braking. The actuator functions norm­
ally 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.
G M C S E R V I C E M A N U A L
Installation
fire wall and connect air lines.
to instrument panel with screw s.
to fire wall.
LIMITING VALVE REPLACEMENT
Removal
Remove valve.
Installation
DD3 BRAKE ACTUATOR SYSTEM
Standard air-operated chambers are used at the
front wheels.
SERVICE BRAKE OPERATION
works. With the handle of the push-pull control
valve pushed in, air pressure from the parking
reservoir is delivered through the push-pull con­
trol valve to the control port of the inversion valve.
From there it is routed to the lock port of the DD3
brake actuator. Air pressure, actihg on the actua­
tor piston moves it forward to contact the rollers
which roll up the ramp of the piston holding them
away from the push plate shaft. This allows the
push plate and shaft to move freely in both direc­
tions for normal service brake application and
release.
PARKING OPERATION
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 actuator 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
1. Position valve in instrument panel or on
2. On tilt cab models fasten valve and plate
3. On other models, fasten valve and bracket
4. Start engine and build up air pressure.
5. Check for leaks.
6. Check operation of brakes.
1. Block vehicle wheels.
2. Drain air from brake system .
3. Disconnect air lines.
4. Remove bolts which fasten valve to vehicle.
1. Connect air lines.
2. Position valve and fasten with bolts.
3. Start engine and build up air pressure.
4. Check for leaks.
5. Check operation of brakes.
Figure 19 shows schematically how the system
To park, the handle of the push-pull control