GMC 1990 Light Truck Repair Manual page 340

5-4 BRAKES
COMPOSITE MASTER CYLINDER
The Delco-Moraine Composite Master Cylinder has
identifying information stamped into the front outlet tube.
The first and second digits are the build code. The third
digit indicates the year it was built (a 5 means 1985 or a 6
means 1987). The last digits indicate the day it was built
(271 means it was the 271st day of the production year).
NOTICE: Do not hone the master cylinder bore.
When the brake master cylinder is overhauled,
it is recommended that the cylinder body be
replaced rather than "cleaned up" by honing
the bore. The master cylinder has a hard, highly
polished " bearingized" surface, which is
produced by diamond boring followed by ball or
roller burnishing under heavy pressure. Honing
will destroy this surface which will cause rapid
wear of rubber cups.
NOTICE: Do not use kerosene, gasoline, or any
other unapproved solvents for cleaning or
flushing master cylinder and components. The
use of these as solvents or any other with a
trace of mineral oil will damage rubber parts.
Remove or Disconnect (Figures 9 and 10)
1. Cover (1).
2. Diaphragm (2).
• Drain all the brake fluid from the reservoir.
3. Reservoir (3) and grommets (4) (figure 11).
• Clamp the mounting flange of the cylinder in a
vise and pry the reservoir off with a bar.
4. Snap ring (11).
5. Primary piston assembly (12).
CAUTION: In the following step if air pressure is
used to remove the secondary piston, place the
open end of the cylinder bore approximately 25
mm (1-inch) from a padded workbench or other
surface to catch the piston when it comes out
of the bore. Apply low air pressure very
carefully to ease the piston out of the bore.
Never point the open end of the bore at anyone
when applying air pressure. The piston may
come out of the bore with considerable force
and cause personal injury.
6. Secondary piston (9).
• With the rear port plugged apply a small
amount of air pressure to the front port.
7. Seals (8 and 10).
8. Spring retainer (7) and spring (6).
»3*| Clean
• All the metal parts in denatured alcohol.
• All the rubber parts in clean brake fluid.
• Stained or discolored cylinder bore with crocus
cloth.
MASTER CYLINDERS
The Delco-Moraine Cast Iron Master Cylinder has
identifying information stamped into the front outlet tube.
The first and second digits are the build code. The third
digit indicates the year it was built (a 5 means 1985 or a 7
means 1987). The last digits indicate the day it was built
(271 means it was the 271st day of the production year).
+ + Remove or Disconnect (Figure 14)
Inspect
Diaphragm for cuts, cracks, or a swollen condition.
Cylinder bore for scoring or corrosion. If corrosion is
present, replace the cylinder. Do not attempt to hone
the bore.
Reservoir for cracks.
Install or Connect (Figures 9 and 10)
• Lubricate the grommets, seals, and cylinder bore
with clean brake fluid.
1. Spring (6) and spring retainer (7).
2. Seals (8 and 10) onto the secondary piston (9).
3. Secondary piston (9).
4. Primary piston assembly (12).
5. Snap ring (11).
• The primary piston must be compressed when
installing the snap ring.
6. Grommets (4).
7. Reservoir (3) (figures 12 and 13).
• Press on the body while using a rocking
motion.
8. Diaphragm (2) into the cover (1).
9. Cover (1).
CAST IRON MASTER CYLINDER
NOTICE: Do not hone the master cylinder bore.
When the brake master cylinder is overhauled,
it is recommended that the cylinder body be
replaced rather than "cleaned up" by honing
the bore. The master cylinder has a hard, highly
polished " bearingized" surface, which is
produced by diamond boring followed by ball or
roller burnishing under heavy pressure. Honing
will destroy this surface which will cause rapid
wear of rubber cups.
NOTICE: Do not use kerosene, gasoline, or any
other unapproved solvents for cleaning or
flushing master cylinder and components. The
use of these as solvents or any other with a
trace of mineral oil, will damage rubber parts.
1. Cover (22).
2. Diaphragm (23).
• Drain all the brake fluid from the reservoir.
3. Snap ring (31).
4. Primary piston assembly (30) (figure 15).