GMC 1976 ZEO 6083 Maintenance Manual page 246

LOW PRESSURE
SECONDARY SEAL
HIGH PRESSURE
SECONDARY SEAL
RETAINER RING
Figure 32-Master Cylinder (Exploded View)
ASSEMBLY (FIGURE 32)
If the brass tube inserts were removed, place the
master cylinder in a vise so that the outlet holes are
up . Position the new brass tube inserts in the outlet
holes, making sure they are not cocked . The recom-
mended method of seating these inserts is to thread
a spare brake line tube nut into each outlet hole and
turn the nuts down until the insert bottoms.
(Remove the tube nut and check the outlet hole for
loose brass burrs, which might have been turned up
when the insert was pressed into position .)
Each vehicle application of these cylinders is de-
signed to produce the correct displacement of fluid
from both the front and rear chambers under nor
mal, failed and partially failed conditions . Delco Mo-
raine dual cylinders are designed so that this variable
displacement requirement is controlled within each
bore size by the dimensions A and C on the second-
ary piston .
Because the pistons vary in length, it is necessary
to mark them with identification rings. It is impera-
tive that exact replacements be made when servicing
the master cylinders.
With all of the variables to be found in master
cylinders, which look similar externally, it is impor-
tant that the complete assemblies be properly identi
fied . For this purpose a two-letter metal stamp will
be found on the end of each master cylinder . This
two-letter stamp indicates the displacement capabili-
ties of a particular master cylinder . It is, therefore,
mandatory that when master cylinders are replaced,
they are replaced with cylinders bearing the same
two-letter stamp.
BRAKES 5- 2 7
1 . Place new secondary seals in the two grooves
in the flat end
of
the secondary piston assembly . The
seal which is nearest the flat end will have its lips
facing toward this flat end. The seal in the second
groove should have its lips facing toward the end of
the secondary piston which contains the small com-
pensating holes.
2. Assemble a new primary seal and primary seal
protector over the end of the secondary piston oppo-
site the secondary seals, so that the flat side of the
seal seats against the flange of the piston which con-
tains the small compensating holes.
3 . In order to insure correct assembly of the pri-
mary piston assembly, a complete primary piston
assembly is included in the repair kits.
4. Coat the bore of the master cylinder with clean
brake fluid . Coat the primary and secondary seals on
the secondary piston with clean brake fluid. Insert
the secondary piston spring retainer into the second-
ary piston spring . Place the retainer and spring down
over the end of the secondary piston so that the
retainer locates inside the lips of the primary seal .
5. Holding the master cylinder with the open end
of the bore up or down, push the secondary piston
into the bore so that the spring will seat in against the
closed end of the bore . Use a small wooden rod to
push the secondary piston to seat .
6. Place the master cylinder in a vise with the
open end of the bore up . Coat the primary and sec-
ondary seals on the primary piston with clean brake
fluid. Push the primary piston, secondary piston stop
first, into the bore of the master cylinder . Hold the
piston down and snap the lock ring into position in
the small groove in the I.D . of the bore .
7. Continue to hold the primary piston down .
This will also move the secondary piston forward
and will insure that the secondary piston will be
forward far enough to clear the stop screw hole,
which is in the bottom of the front fluid reservoir.
The stop screw is now positioned in its hole and
tightened to a torque of 25-40 inch-pounds.
8 . Install a new reservoir diaphragm in the reser-
voir cover and install the cover on the master cylin-
der. Assemble the bail wires into position to retain
the reservoir cover.
WHEEL CYLINDER OVERHAUL
DISASSEMBLY
1 . Pull boots from cylinder ends and discard
boots.
2 . Remove and discard pistons and cups .