Chevrolet 10 Series 1968, Service Manual, Page: 251 / 713

BRAKES 5-18
2 .
3.
4.
5.
1. The two-letter identification stamp on the end of the
master cylinder indicates the displacement capa­
bilities of that particular cylinder. Master cylinders
should only be replaced with another cylinder bear­
ing the same two-letter identification.
2. The length of the component pistons in the master
cylinders are critical factors in displacement capa­
bilities of a particular master cylinder. These
pistons are coded, using rings or grooves in the
shank of the piston. It is mandatory that when pistons
are replaced, the replacing piston must contain the
same identification marks and the same contour at
the push rod ends as the piston which was removed.
3. Delco Moraine dual master cylinders, used with
drum-type brakes, contain a rubber check valve and
check valve spring in each outlet boss. Follow in­
structions given below under disassembly when re ­
placing check valves.
Removal
1. Wipe main cylinder and lines clean with a clean
cloth. Place dry cloths below main cylinder area to
absorb any fluid spillage.
Disconnect hydraulic lines at main cylinder. Cover
line ends with clean lint-free material to prevent
foreign matter from entering the system.
Unbolt and remove the main cylinder.
Remove the main cylinder mounting gasket and boot.
Remove the main cylinder cover and dump-out the
fluid. Pump the remaining fluid from the cylinder by
depressing the push rod.
Disassem bly (Fig. 32)
1. Clamp main cylinder in a bench vise.
2. Remove secondary piston stop bolt from bottom of
front fluid reservoir.
3. Remove the snap ring retainer and primary piston
assembly. Remove the secondary piston, piston
spring, and retainer by blowing air through the stop
bolt hole. (If no air is available, a piece of wire may
be used. Bend approximately 1/4 inch of one end into
a right angle, hook the secondary piston and pull it
out.)
4. Position main cylinder in vise with outlet holes
facing up.
Drill a 13/64 inch hole through both check valve
seats.
Tap out both seats using a 1/4 x 20 tap.
Install a spare brake line tube nut in the outlet hole.
Place a flat washer on a one inch screw (threaded to
screw into tapped hole), and thread screw into
threaded hole in tube seat. Hold the screw to keep it
from turning and back out the tube nut. This will re ­
move the tube seat.
8. Repeat Step 8 above on second tube seat.
9. Remove the check valves and springs from the cavi­
ties beneath the tube seats.
10. Remove the primary seal, primary seal protector,
and secondary seals from the secondary piston. Re­
move the piston extension screw securing the p ri­
mary piston spring to the primary piston. Remove
the spring retainer, primary seal, primary seal
protector, and secondary seal from the primary
piston.
C leaning and Inspection
1. Remove main cylinder casting from vise and inspect
5.
6
.
7.
F ig . 32—Bendix Dual Main C y lin d e r—Exploded V iew
1. Cover
2. Diaphragm
3. Rear Piston
Assembly
4. Snap Ring
5. Secondary Cups
6. Front Piston
7. Cup Protector
8. Primary Cup
9. Cup Retainer
10. Front Piston
Return Spring
11. Piston Top Screw
12. “ O ” Ring
13. Tube Seat
Inserts
14. C h eck V alve s
15. C h eck V a lv e
Springs
the bore for corrosion, pits, and foreign matter. Be
sure that the outlet ports are clean and free of brass
cuttings from the tube seat removal operation.
2. Inspect the fluid reservoirs for foreign matter.
Check the bypass and compensating ports to the cyl­
inder bore to insure that they are not restricted. Do
not use wire to check ports.
NOTE:
Before washing parts, hands must be
clean. Do not wash hands in gasoline or oil
before cleaning parts. Use soap and water only.
3. Use Declene or equivalent to clean all metal parts
thoroughly. Immerse parts in the cleaning fluid and
brush with hair brush to remove foreign matter.
Blow out all passages, orifices, and valve holes. A ir
dry the parts and place on clean paper or lint-free
clean cloth.
NOTE: Be sure to keep parts clean until re ­
assembly. Rewash parts, if there is any oc­
casion to doubt cleanliness.
4. Check pistons for scratches or other damage; re­
place if necessary.
Assem bly (Fig. 32)
Use care when reassembling the main cylinder check
valves. Improper assembly of the check valve seats will
result in distortion of the seats. If this occurs, there will
be no check valve action and a loss of brake pedal travel
will result; the pedal will have to be pumped one or more
times before actual car braking occurs.
1. Place the main cylinder in a vise with the outlet
holes facing up. Place the check valve springs in the
outlet holes. Be sure the springs are seated in the
bottom of the holes. Place new rubber check valves
CHEVROLET TRUCK SERVICE MANUAL