GMC 4000 Series Service Manual page 226

test is probably caused by worn or deteriorated
inlet valve seal or by binding or corrosion between
the exhaust valve disc and the valve cage, prevent­
ing the inlet valve from fully closing.
Make and hold a high pressure application.
Coat the exhaust port and the top of the valve with
soap suds. No leakage is perm issible.
Leakage evidenced by these tests may be due
to worn or deteriorated exhaust valve or leaking
piston seals.
PREVENTIVE MAINTENANCE
BRAKE PEDAL
No regular, periodic maintenance is required.
At the time of each ch assis lubrication it is well
to check pedal operation.
1. Tilt Cab Models. If the pedal does not move
"free and easy," lubricate the pedal to bracket pin
Metal tubing and flexible hose are used to
connect the various units in the air brake system .
Service instructions for both types follow:
METAL TUBING
Metal lines are of annealed copper tubing with
three-piece compression type fittings. Flared type
fittings should never be used in air brake system s.
Connections should be tested at least every 5,000
m iles and tightened or replaced, if necessary.
When replacing metal tubing, tubing must be free
of burrs, copper cuttings, and dirt. Blow tubing out
with compressed air. Any of the above mentioned
particles will destroy sealing seats in air control
units. New tubing must be of the same size as the
old tubing.
Always use a new sleeve when replacing tub­
ing. When tightening tube connector nuts, tighten
to torque listed below to assure an airtight con­
nection. Overtightening will cause leakage. Apply
S.A.E. #10 lube oil to ball sleeves, tubes, and male
threads, then torque to minimum value and check
for leaks. If leaking, back off tube nut approximate­
ly % turn and retorque to higher than minimum
value.
Tubing Size Torque Inch-Pounds
1/4 Inch
3/8 Inch
1/2 Inch
5/8 Inch
FLEXIBLE HOSE
Flexible hose is used at each brake chamber,
between cab and frame, and at trailer connections
where it is impossible to use metal tubing due to
constant flexing during vehicle operation. Hose
G M C S E R V I C E M A N U A L
AIR LINES
30 to 50
30 to 50
90 to 115
100 to 125
and the push rod to pedal pin with engine oil. Use
oil sparingly. Do not drip oil on pedal pad.
2. Conventional Cab Models. Lubricate hinge
pin and roller pin with engine oil. Use oil sparing­
ly. Do not drip oil on pedal pad.
3. Cowl Models. Lubricate nylon bushings on
clutch and brake pedal shaft and sleeve sparingly
with engine oil. Also oil push rod to pedal bolt.
Do not drip oil on pedal pad.
BRAKE APPLICATION VALVE
It is recommended that every year, or after
50,000 m iles, whichever occurs first (and also
dependent upon the type of operation and operator
experience) that the application valve be removed
from the vehicle, disassembled and various com ­
ponents inspected for wear or deterioratioi).. In­
stall new parts where they are found to be worn or
damaged. This applies to all valves on all models.
connections should be tested at least every 5,000
m iles and tightened or replaced if necessary. Any
hose which is chafed, worn, or kinked should be
replaced.
SERVICEABILITY TESTS
1. Operating Test
If any trouble symptoms, such as slow brake
application or release, indicates a restricted or
clogged air line, disconnect the suspected tube or
hose at both ends and blow through it to make sure
the passage is clear. Inspect tubing and hose for
partial restriction such as would be caused by
dents or kinks. If such condition is found, tubing
or hose should be replaced.
2. Leakage Test
With air system fully charged and brake ap­
plied, coat all tubing and hose connections with
soap suds to check for leakage. No leakage is per­
m issible. Leakage is sometimes corrected by
tightening the connection. If this fails to correct
leakage, new fittings, metal tubing, or flexible
hose must be installed.
TRAILER CONNECTIONS
On vehicles equipped with trailer connections,
two air lines are used to connect the truck brake
system to the trailer brake system . One of these
lines is the "Service" line and the other is the
"Emergency" line. The emergency line delivers
constant air pressure to the trailer air tank and
operating valve, while the service line delivers
air pressure to trailer brakes only while the trailer
brakes are being operated, either by means of the
Sec. 5B
Page 219
AIR BRAKES