Bleeding Brakes - GMC 4000 Series Service Manual

remove the cover from the access hole and insert
a screwdriver. Place a corner of the screwdriver
blade in the hole in the adjusting lever and push
Use only Hydraulic Brake Fluid recommended
in LUBRICATION (SEC. 0). When other than rec­
ommended fluid has been used, drain and flush the
entire hydraulic system , using clean alcohol or a
hydraulic brake system cleaning fluid. D isassem ­
ble, clean, and inspect hydraulic units. Replace all
rubber parts. Refill system with recommended
fluid.
The need for bleeding air from system is gen­
erally indicated by a springy, spongy pedal action.
The presence of air in system is a result of low
fluid level in master cylinder, or of some part of
the system having been disconnected. Bleeder
valves are provided on the power cylinder (some
models), master cylinder (some models) and at
wheel cylinders. Type "F" and type "FA" front
brakes have two cylinders per wheel and each has
a bleeder valve. Type "FR-3" and type "FR-3A"
rear brakes have two cylinders per wheel with a
bleeder screw at the top cylinder only. Duo-Servo
front brakes have one cylinder per wheel and each
has a bleeder screw. Twin-Action rear brakes
have two cylinders per wheel with a bleeder screw
at the top cylinder only. Refer to figure 7 for bleed­
ing sequence by truck model.
Master cylinder is accessible under the hood
on conventional models, through access door in
seat riser on tilt cab models and through access
door in floor on "P" model. Bleeder valves at
wheel cylinders are accessible at inner sides of
the backing plates.
It is recommended that brake system is bled
in a definite sequence to obtain best result. Figure
7 illustrates various combinations of brake equip­
ment used, with bleeder valves numbered in the
recommended sequence in which they should be
bled.
IMPORTANT: Note that neither master cylin­
der nor power cylinder on some models is equip­
ped with a bleeder valve. If bleeding at the wheels
only does not produce satisfactory results, it will
be necessary to disconnect line from master cyl­
inder and bleed it through output port. Then re­
connect line and disconnect power cylinder output
line and bleed. Reconnect line and check oper­
ation of brakes. If still not satisfactory, rebleed
wheel cylinders in sequence shown.
There are two methods of bleeding hydraulic
G M C S E R V I C E M A N U A L
the lever away from the star wheel. Using a brake
adjusting tool, back off the star wheel. See figures
4 and 5.

BLEEDING BRAKES

brake system s used on these vehicles; pressure
bleeding and manual bleeding. Both are acceptable
and adequate (except on split system s) but p res­
sure bleeding is recommended, if equipment is
available.
HYDRAULIC BRAKES
CAUTION: Before starting pressure
bleeding operations, stop engine and des­
troy vacuum in system before opening any
bleeder valve. _____
Figure 6 — Brake Bleeding Tool for
Split System Master Cylinder
Sec. 5A
Page 165
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