GMC 4000 Series Service Manual page 192

least 26 inches of mercury vacuum. Let the engine
idle. The pump should maintain at least 20 inches
of mercury vacuum with the engine idling. If the
pump does not maintain the vacuum values given
above, inspect the pump for belt slippage or lack
of lubrication. Also inspect the vacuum lines and
fittings for leaks. If vehicle is not equipped with
vacuum gauge, temporarily connect one in intake
line.
BINDING IN THE PUMP
After the pump has been running for several
minutes, stop the pump, loosen the belt adjusting
device and remove the belt. With the vacuum port
of the pump open to atmosphere, rotate the pump
in the direction indicated on the front end plate,
and check for points at which the pump tends to be
binding. If binding occurs, the pump should be re­
moved from the installation, disassembled, and
inspected for faulty parts.
OIL LEAKS
After the pump has been operating for several
minutes, inspect between the housing and the end
plates for signs of oil leakage. This check should
be made both with the vacuum port open and closed
to atmosphere.
VACU U M PUMP REPLACEMENT
(Refer to Figure 24)
REMOVAL
1. Loosen adjusting arm bolt and nut.
2. Loosen tension on drive belt and remove
belt from drive pulley.
3. Disconnect air intake hose.
4. Disconnect oil inlet hose.
5. Disconnect oil and air discharge hose.
6. Remove four bolts which secure vacuum
pump to support base.
7. Remove pump and discard gasket.
INSTALLATION
1. Position new gasket on pump base.
Air power assisted hydraulic brakes are
standard equipment on models LA4000 and DLA-
4000. These models use an air power cylinder to
activate the standard type hydraulic brake. A reg­
ular air com pressor is used as the air supply
source.
NOTE: Full air brakes are also available on
these same models, as optional equipment. Refer
to "AIR BRAKES" (SEC. 5B) of this manual.
G M C S E R V I C E M A N U A L
1 Crankshaft Pulley
2 Drive Belt
3 Drive Pulley
4 Adjusting Arm
5 Adjusting Bolt
6 Air Intake
7 Oil Inlet Line
8 Pump Body
bolts.
tension (120-130 pounds (new belt) or 80-90pounds
(used belt)) using a strand tension gauge placed at
the center of the greatest belt span.
AIR POWER CYLINDERS
DLA-4000 models in place of a vacuum-assisted
power unit. With the air power unit, the hydraulic
brake system remains unchanged.
elements:
HYDRAULIC BRAKES
9 Body to Base Bolt
10 Mounting Base
11 Base to Bracket Bolt
12 Mounting Bracket
13 Air and Oil
14 Bracket to Engine
Figure 2 4 — Vacuum Pum p Installed (Typical)
2. Position pump on base and secure with four
3. Connect oil and air discharge hose.
4. Connect oil inlet hose.
5. Connect air intake hose.
6. Install belt on drive pulley and pull to proper
7. Tighten adjusting bolt and nut.
AIR POWER BRAKE SYSTEM
OPERATION
An air-operated power unit is used on LA and
The air power unit consists of three basic
1. An air cylinder which includes a cylinder,
Sec. 5A
Page 185
Discharge Hose
Bolt
T-2943