Chevrolet 10 Series 1968, Service Manual

Page: 338 / 713

BRAKES 5-95
2. Install piston assembly in cap assembly.
3. Position rubber washer in pressure housing at bottom
of internal threads. Thread cap and spring assembly
into pressure housing and tighten firm ly against rub­
ber washer. This may be done with "strap" type
wrench (See Figures 137 and 138) or by putting pres­
sure housing in vise and using a wrench on hex end of
cap.
4. Position diaphragm over plate rod, then install pres­
sure housing and cap assembly.
NOTE: Diaphragm should fit evenly between
pressure housing and non-pressure housing.
Align marks made on housings at disassembly
to insure proper air port location.
5. Pull clamping ring over flange of pressure and non­
pressure housings. Install clamp bolt and nut and
tighten firm ly.
6. Connect all air lines and build up air pressure in
system to normal operating pressure. Push parking
brake knob in to admit air to outer brake chamber.
EMERGENCY AIR BRAKES
GENERAL DESCRIPTION
7. Turn release bolt counter-clockwise as far as possible
(approx. 18 turns) to release spring (See Figure 135).
This will leave the parking brake chamber in operat­
ing condition.
NOTE: It is best to have at least 60 psi of air
in chamber to hold spring tension off nut so a
better "fe e l" is obtained when the nut does bottom.
Full emergency air brake equipment is available on all
C, M and S60 series trucks for 1968. Two completely
separate systems, primary and secondary, combine to
function as the service brakes. In the event of failure of
one system, the remaining system continues to operate
until the difficulty is corrected.
The primary system, on a single rear axle trucks, con­
sists of a large diaphragm in the DD2 safety actuators,
while the secondary system includes the front brakes and
smaller, second diaphragm of the DD2 chambers.
Automatic and manual actuation of the secondary system
is provided. Secondary system brake chambers are auto­
matically applied whenever air pressure falls below 45
psi. These brakes cannot be released until pressure in
the secondary system returns to a satisfactory operating
level.
Valve location and piping has been simplified this year.
A ll valves are now located in the cab or engine compart­
ment (fig. 139).
There are two air tanks used on base installations, a
primary and secondary. The secondary tank is a split
tank a "dry" and "wet" side. Air heated because of com­
pression enters the "wet" side of the secondary tank where
it expands and cools. This cooling action allows the water
vapor in the air to condense and settle out. The "dryer"
air passes through a one-way check valve mounted be­
tween the "wet" and "dry" side of the secondary tank into
the dry side. The dry side of the secondary tank feeds
the secondary circuit.
The purpose of both tanks is to provide a place to store
compressed air, so there will always be ample supply for
immediate operation of the brakes and consecutive appli­
cations thereafter. Even through some standard air brake
installations use two tanks, this installation differs in
that each tank supplies air to separate circuits indepen­
dent of one another. Although the compressor feeds both
tanks, the tanks are connected in such a manner (by using
one-way check valves) they cannot feed back into each
other (fig. 140).
MAINTENANCE AND ADJUSTMENTS
System Checking
At regular intervals the brake system should be tested
by putting it through its various modes of operation as
described in the preceding text. This same procedure
should also be used when isolating system malfunctions.
System Leakage Test
Install an air pressure gauge in secondary circuit (use
dash gauge for primary circuit readings).
Run engine until governor cuts out. Stop engine. Wait
until air pressure stabilizes. With brakes released, gauge
pressure drop of either system should not exceed 2 pounds
per minute for single axle vehicles or 3 pounds per min­
ute for tandem axle vehicles. Make a full brake applica­
tion and allow pressure to stabilize. Pressure drop of
either system should not exceed 3 pounds per minute for
single axle vehicles or 4 pounds per minute for tandem
vehicles. Check system connections and applicable units
if leakage in either test exceeds the permissible rate.
Also remember during operation that the low pressure
switches and dash gauge are incorporated into the system
in such a manner that air leakage can be immediately
traced to the primary or secondary system. If both the
CHEVROLET TRUCK SERVICE MANUAL