GMC 4000 Series Service Manual page 185

Sec. 5A
Page 178
HYDRAULIC BRAKES
BOOT.
Figure 16— Duo-Servo W heel Cylinder
Inspection and Repair
1. Repair kits are available which contain the
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. Clearance should be within
0.002" to 0.004". Replace cylinder if clearance
exceeds 0.004". Refer to figure 15.
Assembly
Before assembling wheel cylinder, be sure
each part has been cleaned in denatured alcohol.
1 Connecting Rod
2 Seal
3 Wheel Cylinder Body
4 Piston
Figure 17 -T w in -A c t io n W heel Cylinder
G M C S E R V I C E M A N U A L
5 Cup
6 Connector Insert
7 Spring Assembly
T-2699
CAUTION: DO NOT use kerosene or
gasoline for cleaning wheel cylinder parts.
Dip each internal part in hydraulic brake
fluid before assembling.
Insert pistons, cups, and spring into cylinder
bore. Assem ble push rods in boots and install boots
over ends of cylinder. Install bleeder valve.
TYPES "FR-3" AND "FR-3A" (Fig. 14)
Disassembly
Pull boots off ends of cylinders and remove
push rods from boots. Push pistons, cups, and
spring out of cylinder. Remove bleeder valve from
upper cylinder.
Inspection and Repair
1. Repair kits are available which contain the
parts to be replaced when overhauling wheel cyl­
inders.
IMPORTANT: Only genuine GMC parts should
be used. Non-genuine parts may deteriorate and
swell in a short time, resulting in faulty brake
operation.
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 cor­
rosion are too deep to be polished out, replace
cylinder.
3. Check fit of new pistons in cylinder bore,
using a feeler gauge as shown in figure 15. Clear­
ance should be within 0.001" to 0.005" on "FR-3"
type. 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. Install pistons, piston cups, and spring,
with cup lips toward inside of cylinder.
2. Assemble push rods and boots, then install
on cylinder. Seat boots evenly in cylinder grooves.
Align push rod slots as shown in figure 14.
3. Install bleeder valve in upper cylinder.
TWIN-ACTION (Fig. 17)
Disassembly
Pull boots off ends of cylinder and remove
push rods from boots. Push pistons, cups, and