GMC 4000 Series Service Manual page 184

TYPE " F" A N D TYPE " F A " - F R O N T
TYPE " FR -3" A N D TYPE " F R -3 A " - R E A R
1 Boot
2 Brake Shoe Guide
3 Piston
4 Piston Cup
5 Cup Filler
Figure 14— W heel Cylinders Used with Type
"F", "F A ", "FR-3", and "FR -3A" Brakes
1. Position heat shields on cylinders, place
wheel cylinders on backing plate, and attach with
cap screw s and lock washers. Tighten cap screws
firm ly. Install dust shield on upper end of each
cylinder.
2. Install connecting tube between the two cyl­
inders and tighten connections firmly. Connect
brake tube to inlet opening in rearward cylinder
at inner side of backing plate. Install bleeder valve
in threaded opening in forward cylinder.
3. Install brake shoes as directed under "Brake
Shoe Installation." Bleed brake system as directed
under "Bleeding Brakes."
WHEEL CYLINDER REPAIR
TYPES "F" AND "FA" (Fig. 14)
Disassem bly
Pull boot off end of cylinder, then remove boot
and piston. Pull boot off piston. Brake shoe guide
is pressed into piston and cannot be removed. Re­
move piston cup, cup filler, and spring from cyl­
inder. Remove bleeder valve from cylinder.
Inspection and Repair
1. Repair kits are available which contain the
G M C S E R V I C E M A N U A L
6 Piston Spring
7 Cylinder
8 Brake Shoe Anchor
Slot
9 Push Rod
T - 4 5 6 6
Figure 15— Checking Fit of Piston in Wheel Cylinder
parts to be replaced when overhauling wheel cyl­
inders. Refer to Parts Book for repair kit part
numbers.
2. Inspect cylinder bore for scores, scratches,
or corrosion. Light scratches or slightly corroded
spots may be polished out with crocus cloth. Never
use emery cloth or sandpaper. If scratches or
corrosion are too deep to be polished out, replace
cylinder.
3. Check fit of new pistons in cylinder bore,
using a feeler gauge (fig. 15). Clearance should be
within 0.0025" to 0.0065" on "F" and "FA" type
cylinders. Replace cylinder if clearance exceeds
the maximum.
Assembly
Before assembling wheel cylinder, be sure
each part has been cleaned in denatured alcohol.
CAUTION: DO NOT use kerosene or
gasoline for cleaning wheel cylinder parts.
Dip each internal part in hydraulic brake
fluid before assembling.
1. Insert piston spring, cup filler, and cup
into cylinder bore. Cup filler bumper and cup lip
must face closed end of cylinder.
2. Assem ble boot on piston, making sure the
boot snaps over the brake shoe guide.
3. Install piston and boot, inserting piston into
cylinder and engaging boot lip in groove in edge of
cylinder. Position brake shoe guide so slot in guide
is parallel with flat mounting surface of cylinder.
DUO-SERVO (Fig. 16)
Disassembly
Pull boots off ends of cylinder and remove
push rods from boots. Push pistons, cups, and
spring out of cylinder. Remove bleeder valve from
cylinder.
Sec. 5A
Page 177
HYDRAULIC BRAKES