GMC 4000 Series Service Manual page 225

Sec. 5B
Page 218
AIR BRAKES
2. To install pedal, valve and support bracket
as an assembly, position in cab and attach assem ­
bly to cab with bolts.
3. Connect exhaust hose to valve exhaust port.
4. Connect air lines to valve.
5. Start engine and build up air pressure in
brake system .
6. Check for leaks.
7. Test operation of brakes.
NOTE: Various item s of special or optional
equipment are often assembled in outlet ports of
the valve, such as low air pressure switch, double
check valve, stop light switch, etc. These items
may be removed separately from the valve by d is­
connecting proper air lines, without the necessity
of removing the valve from the vehicle. They may
also be taken out WITH the valve as an assembly,
if desired.
CONVENTIONAL CAB MODELS
Removal
1. To remove pedal only, remove cotter pin
and drive out pedal to bracket pin.
2. To remove pedal and valve as an assembly
block vehicle wheels or hold by some means other
than air brakes.
3. Drain air pressure from brake system .
4. Disconnect air lines from valve both in cab
and in engine compartment.
5. From engine side of cowl, remove one bolt
and two stud nuts which fasten valve to cowl.
6. Remove pedal and valve assembly from
inside cab.
Installation
1. Place pedal and valve assembly in position
on cowl inside cab.
2. From engine compartment side of cowl,
install one bolt and two stud nuts to fasten valve
to cowl.
3. Connect air lines to valve, both in cab and
in engine compartment.
4. Start engine and build up air pressure to
operating level.
5. Check for leaks.
6. Test operation of brakes.
NOTE: Various item s of special or optional
equipment are often assembled in outlet ports of
the valve, such as low air pressure switch, double
check valve, stop light switch, etc. These items
may be removed separately from the valve by d is­
connecting proper air lines, without the necessity
of removing the valve from the vehicle. They may
also be taken out with the valve as an assembly,
if desired.
G M C S E R V I C E M A N U A L
"COWL" MODELS
Removal
to remove the clutch pedal. Refer to "HYDRAULIC
BRAKES" (SEC. 5A) under heading of "Brake Pedal
Replacement (All Except Tilt Cab)"for instructions.
or hold by some means other than air brakes.
valve from mounting bracket, or remove two bolts
from mounting bracket and remove valve, bracket
and boot from firewall as an assembly.
Installation
and boot) in position and secure with bolts.
brake system .
equipment are often assembled in outlet ports of
the valve, such as low air pressure switch, double
check valve, stop light switch, etc. These items
may be removed separately from the valve by d is­
connecting proper air lines, without the necessity
of removing the valve from the vehicle. They may
also be taken out WITH the valve as an assembly,
if desired.
OPERATION TESTS
using an accurate test gauge connected into one of
the air lines leading to the brake chambers. De­
p ress the pedal to several positions between the
fQlly released and fully applied positions and check
the delivered pressure on the test gauge to see
that it varies proportionately with the movement
of the pedal. When the pedal is fully applied, the
reading on the test gauge should be approximately
that of full reservoir pressure. The reading on the
test gauge should fall off to zero when application
is released. Also, check pressures registered on
the dash gauge. These should agree with test gauge
readings within 5 pounds.
LEAKAGE TESTS
haust port or end of exhaust tube for leakage. No
leakage is perm issible. Leakage evidenced by this
1. To remove pedal only, it is also necessary
2. To remove valve only, block vehicle wheels
3. Drain air pressure from brake system .
4. Disconnect air lines from valve.
5. Remove three bolts from valve and remove
1. Place valve (or valve, mounting bracket
2. Connect air lines to valve.
3. Start engine and build up air pressure in
4. Check for leaks.
5. Test operation of brakes.
NOTE: Various items of special or optional
APPLICATION VALVE
SERVICEABILITY TESTS
Check the delivery pressure of the brake valve
With the valve fully released, check the ex­