Hydraulic Lines - GMC 4000 Series Service Manual

spring out of cylinder. Remove cups from pistons.
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. 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 cor­
rosion 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.
CAUTION: DO NOT use kerosene or
gasoline for cleaning wheel cylinder parts.
Dip each internal part in hydraulic brake
fluid before assembling.
Hydraulic brake system units are intercon­
nected by flexible hose and special metal tubing.
Flexible hose is used between master cylinder (on
cab) and frame connection, between frame and
front wheel cylinders, and between frame and rear
axle brake line. Whenever hydraulic lines have
been disconnected for any reason, brake system
must be bled, after connecting lines, as directed
under "Bleeding Brakes."
FLEXIBLE HOSE
At front wheels on type "F" and type "FA,"
hose fitting is threaded into wheel cylinder connec­
tor, with a copper gasket used between shoulder
on hose fitting and connector. On Duo-Servo a con­
nector is not used. Fitting at other end of hose is
inserted through hole in frame and secured by a
toothed lock washer and nut or by a spring lock.
Brake tube connector or tee fitting threads into
end of hose fitting.
At rear axle, fitting at one end of hose is
threaded into axle tee, with a copper gasket used
to seal the connection. Other end of hose is insert­
ed through frame bracket and secured by a toothed
lock washer and nut or by a spring lock. Brake
G M C S E R V I C E M A N U A L
of cup will be toward flat end of piston. Use special
tool (J-4705) to guide cup over piston (fig. 18).
This applies to old style assem blies only.
bead on boot engages groove in push rod. Install
boots and push rods on cylinders, engaging bead on
outer edges of cylinder housing.

HYDRAULIC LINES

tube connector or tee fitting threads into end of
hose fitting.
frame bracket, then unscrew hose fitting from
wheel cylinder or rear axle tee. When installing
hose, always use a new copper gasket at wheel
cylinder and rear axle tee. When frame end of hose
is secured by a nut, always hold hose fitting with a
wrench while tightening nut to prevent twisting
hose.
METAL TUBING
always use special metal tubing which is designed
to withstand high pressure and to resist corrosion.
Ordinary copper tubing is not satisfactory for use
as hydraulic brake lines. When replacing tubing,
always use the same size as that removed.
TUBE FLARING
flaring tool must be used. When using tool, in­
structions furnished by the tool manufacturer
should be followed. Always inspect newly formed
flares for cracks or malformations which might
cause leaks.
HYDRAULIC BRAKES
Figure 18— Using Special Tool (J-4705) for Installing
W heel Cylinder Cup (Twin-Action Only)
1. Install new cup on each piston so open end
2. Assem ble boots on push rods, being sure
To remove hose, disconnect end at frame or
When necessary to replace metal brake tubing,
In order to insure a proper flare, a special
Sec. 5A
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