Chevrolet Light Duty Truck 1973 Service Manual page 306

5 -4 0 BRAKES
replacement cylinder are recommended for
this operation.
2. Clamp the master cylinder in a bench vise with the
front end tilted slightly down.
CAUTION:
Do not tighten v i s e too t i g h t l y as
damage t o the master cylinder could r e s u l t .
3. Fill both reservoirs with clean brake fluid.
4. Insert a rod with a smooth round end to the
primary piston and press in to compress the piston
return spring.
5. Release pressure on rod. Watch for air bubbles in
the reservoir fluid.
. Repeat Step 5 as long as bubbles appear.
6
7. Reposition master cylinder in vise so that the front
end is tilted slightly up.
. Repeat Steps 4-5-6.
8
9. Install diaphragm and cover on reservoir.
BRAKE COMBINATION VALVE
Metering, Warning and Proportioning (Fig.
73)
NOTE: The brake combination valve is a
non-adjustable, non-serviceable valve. If
defective it must be replaced.
Function
Metering Valve
This section of the valve operates to "hold-off" hydraulic
flow to the front disc brakes until a predetermined
pressure is reached. This "hold-off" action allows the
rear drum brakes to build up sufficient hydraulic
pressure to overcome the force of their retracting
springs. This metering or hold-off valve then provides
for balanced braking.
NOTE: When bleeding the brakes; the pin in
the end of the metering portion of the
combination valve must be held in the open
SWITCH
TERMINAL
FROM MASTER
CYLINDE
TO FRONT
BRAKES
Fig. 73-Combination Valve
FROM MASTER CYLINDER
TO REAR
BRAKES
position (not allowed to close). This can be
accomplished by installing Tool J-23709
under the mounting bolt and depressing the
pin in the end of the valve. Be sure to re­
torque the mounting bolt after removing
Tool J-23709 (fig. 29).
Warning Switch
The warning switch is the pressure differential type. It is
wired electrically to the warning lamp on the instrument
panel to warn the vehicle operator of a pressure
differential between the front and rear hydraulic systems.
Once the switch is activated "on", it will not "reset" until
the defect in the hydraulic system has been repaired.
Hydraulic reset to the "off" position occurs with the
application of equal front and rear pressures.
Proportioning Valve
The function of this valve is prevent premature rear
wheel slide. Line pressure is allowed to increase normally
up to a certain point (determined by vehicle weight and
braking distribution). When the predetermined pressure
is reached, the valve begins to function and limit the
amount of increase in hydraulic pressure passed to the
rear brakes. This prevents the rear brakes from locking
up before the full effective braking effort is produced by
the front disc brakes.
NOTE: In the event of "front hydraulic
system failure" the proportioning valve has a
"by-pass" feature that assures full system
pressure to the rear brakes.
Removal
1. Disconnect electrical lead.
2. Place dry rags below valve to absorb any fluid
spillage.
3. Wipe off any dirt and disconnect hydraulic lines
from valve—cover open lines to prevent foreign
matter from entering the system.
4. Remove mounting screws and remove valve.
Installation
1. Make sure new switch is clean and free of lint. If
any doubt exists, wash the switch in clean brake
fluid.
2. Place new switch in position and secure with screws.
CAUTION:
See "Caution" on Page 1 of t h i s
s e c t i o n .
3. Connect hydraulic lines to valve.
CAUTION:
See "Caution" on Page 1 of t h i s
s e c t i o n .
4. Connect switch electrical lead.
5. Bleed the brake system.
LIGHT DUTY TRUCK SERVICE MANUAL