GMC 4000 Series Service Manual page 149

Sec. 4A
Page 142
REAR AXLE AND CONTROLS
Figure 2 7 — Two-Speed Axle Vacuum Shift
C ham ber Installed (Rockwell)
2. Remove ten screws attaching cover to
chamber, then remove cover from body of cham­
ber.
3. Remove nut from end of push rod, then re­
move diaphragm and plates assembly from push
rod.
1 Diaphragm Body
2 Diaphragm Return
Spring
Figure 2 8 — Speedom eter A dapter Shift D ia p h ra g m
G M C S E R V I C E M A N U A L
3 Diaphragm and Rod
Assembly
4 Mounting Bracket
5 Screws
-9 0 8 3
t p m
4. Remove two nuts and lock washers from
studs (fig. 27) attaching chamber mounting flange
to mounting sleeve and carrier. Remove chamber
assembly.
5. Force push rod in to relieve spring load,
then remove snap ring (18) from inside of piston
rod. Pull push rod and spring assembly from piston
rod.
6. Piston rod and shift fork cannot be removed
unless carrier assembly is removed from housing.
Refer to respective axle section for procedure.
ASSEMBLY AND INSTALLATION (Fig. 26)
1. If disassembled, assemble push rod by in­
stalling parts in following order: washer, spring,
washer, spacer, and nut. Stake nut securely to pre­
vent loosening.
2. Position push rod and spring assembly into
piston rod. Compress spring sufficiently to install
snap ring (18).
3. Apply sealing compound on the diaphragm
plates around holes. Position diaphragm between
two plates with diaphragm bead in grooves of plates.
4. Secure plates together with bolt, locking
plate, and nut (19). Make sure that diaphragm is
properly located, then tighten nut just enough to
form a vacuum-tight seal.
5. Install chamber inner shell with flange
over studs (fig. 27). Install lock washers and nuts,
tighten nuts securely.
6. Position diaphragm assembly over push
rod, then install nut. Tighten nut securely.
7. Install cylinder outer shell to inner shell
with diaphragm between shells. Install and tighten
screws firmly.
8. Attach hose in their original location, using
hose clamps.
CHECK VALVE
Check valve, in intake manifold line, is used
to prevent vacuum loss or drop when engine is
accelerated or is pulling under heavy load. Check
valve also effectively retards lo ss of vacuum when
engine is stopped. Refer to "HYDRAULIC BRAKES"
(SEC. 5A) of this manual for service maintenance
information.
AIR CLEANER
An air cleaner is used at atmospheric inlet
line to filter air entering control valve and shift
units.
MAINTENANCE
At regular intervals remove air cleaner and
im merse in cleaning solvent to remove all accum­
ulated dirt or other foreign material. Drain clean­
ing solvent from element. Reinstall dry.