Chevrolet 10 Series 1968, Service Manual

Page: 239 / 713

BRAKES 5-6
3. Place dry rags below the switch to absorb any fluid
spilled during removal of switch.
4. Disconnect four hydraulic lines from connections at
switch. If necessary, loosen line connections at
main cylinder to loosen lines. Cover open line ends
with clean, lint-free material to prevent foreign
matter from entering the system.
5. Remove mounting screw and remove switch from
vehicle.
Installation
1. Make sure new switch is clean and free of dust and
lint. If any doubt exists, wash switch in Declene or
equivalent, and dry with air.
2. Place switch in position and secure to bracket with
mounting screw.
3. Remove protective material from open hydraulic
brake lines and connect lines to switch. If necessary,
tighten brake line connections at main cylinder.
4. Connect switch electrical lead.
5. Connect battery cable.
6. Bleed the brake systems as outlined in this section.
BRAKE WARNING LIGHT CHECKING
PROCEDURE
1. Determine if bulb is functioning by depressing park­
ing brake.
2. Check to make sure that both master cylinder reser­
voirs are full.
3. Open wheel cylinder bleed screw at one rear wheel.
4. Depress brake pedal and hold down. The brake
warning light should come on due to pressure dif­
ference between front and rear systems. Approxi­
mately 200-250 psi differential is needed to operate
brake light.
5. Close bleed screw, then release brake pedal. Refill
rear reservoir, if needed.
NOTE:
Caution should be taken to prevent air
from entering hydraulic system during checks on
switch.
The recommended checking interval is 24 months or
24,000 miles, any time major brake work is done or any
time a customer complains of excessive pedal travel.
MAINTENANCE AND ADJUSTMENT
In any service operation it is extremely important that
absolute cleanliness be observed. Any foreign matter in
the system will tend to clog the lines, ruin the rubber
cups of the wheel and main cylinders and cause inefficient
operation or even failure of the braking system. Dirt or
grease on a brake lining will cause that brake to grab
first on brake application and fade out on heavy brake
application.
BRAKE SHOE ADJUSTMENT
The duo-servo and Twinplex brakes used on all 10 thru
50 series models are equipped with self-adjusting mech­
anisms to compensate for normal facing wear. Sixty
series models are equipped with Wagner manually or
self adjusted brakes; the adjustment is made through an
opening in the flange plate or externally as outlined
below.
Wagner Type “ F”
1. Jack up axle. Place wrench on one adjusting cam
stud to adjust one shoe (fig. 9). Rotate wrench in
direction of forward wheel rotation to decrease lin­
ing to drum clearance. Reduce clearance until brake
drag is felt as wheel is turned in forward direction
by hand.
2. Move wrench slightly in opposite direction until
brake drag is relieved, then move wrench an addi­
tional 7 to 10 degrees to provide running clearance
(7 to 10 degrees is equal to 1 to 1 1/2 inches of
travel at end of an 8-inch wrench).
3. Place wrench on opposite adjusting cam stud and
adjust second shoe by repeating Steps 1 and 2.
Wagner Type “ FR-3”
1. Jack up wheels until clear of floor. Remove adjusting
hole covers from backing plate.
2. At one adjusting slot, insert suitable tool through slot
and engage adjusting wheel (fig. 10). Move tool
handle toward axle to rotate adjusting wheel and
decrease lining clearance until lining drags on drum.
Firmly depress foot pedal to center shoes and again
decrease clearance until drag is felt on drum.
3. Relieve drag by rotating adjusting wheel in opposite
direction. Back off adjustment as follows:
For worn lining—3 notches (clicks)
For new lining—5 notches (clicks)
4. At other adjusting slot, repeat steps 2 and 3 to adjust
other shoe.
5. Install adjusting hole covers in backing plate.
Bleeding Brakes
The hydraulic brake system must be bled whenever any
line has been disconnected or air has in some way
entered the system. A "spongy” pedal feel when the
brakes are applied may indicate presence of air in the
system. The system must be absolutely free of air at all
times. Bleeder valves are provided on power cylinder
(some models), main cylinder (some models) and at
wheel cylinders. Type “ F” front brakes have two cyl­
inders per wheel and each has a bleeder valve. Type
“ FR-3” rear brakes have two cylinders per wheel with a
bleeder valve at the top cylinder only. See Figure 12 for
bleeding sequence.
NOTE:
If main cylinder or power cylinder is
not equipped with a bleeder valve and bleeding at
the wheels does not produce satisfactory results,
it will be necessary to disconnect line from main
cylinder and bleed it through output port. Then
reconnect line and disconnect power cylinder
output line and bleed the power cylinder. Re­
connect line and check operation of brakes.
If still not satisfactory, rebleed wheel cylinders
in sequence shown.
CHEVROLET TRUCK SERVICE MANUAL